Short answer
Spill is light from the screen bouncing onto the subject, so the shoulders, the chin and the hair really are slightly green. Prevent it by moving the subject further from the screen, not overlighting the screen, and adding a back light. Remove what remains by subtracting the screen's hue while keeping the pixel's brightness, which is what a proper spill suppression does. Pulling the saturation down instead removes the colour and the life together, which is why so many keyed subjects have grey hair.
Everybody’s first green screen result has a green edge somewhere on it. It is not a bug in the software and it is not a sign that the screen was the wrong shade of green. It is physics, and like most physics it has both a prevention and a cure.
The green screen guide covers the setup this assumes.
Where it comes from
A green screen is a large surface reflecting a great deal of green light. Anything in front of it is being lit by that light, in addition to whatever lights you set up.
Light falls off with distance, so the effect is strongest where the subject is closest to the screen and where surfaces face it: the tops of shoulders, the sides of arms, the back of the neck, the underside of the chin, the rims of ears, and any hair that is not lying flat.
The pixels really are green. If you pointed a colour meter at that shoulder, it would read greener than the same shoulder in daylight. This matters because it explains why the key cannot fix it: the key’s job is to decide what is background, and a green shoulder is not background.
Prevention beats correction
Every one of these is cheaper than fixing it afterwards.
Move the subject away from the screen. This is the big one. Spill falls off with the square of the distance, so going from half a metre to two metres reduces it by roughly sixteen times. If you change nothing else, change this.
Do not overlight the screen. A screen wants to be evenly lit, not bright. If it is brighter than the subject, it is throwing more light at them than your key light is. Matching the screen roughly to the subject, or leaving it slightly under, is the target.
Light the subject separately and generously. Spill is a proportion. The more of your own light lands on the subject, the smaller the share of green in the total. A well lit subject in front of a modestly lit screen has very little spill.
Add a back light. A light behind and above the subject, aimed at the shoulders and the top of the head, does two jobs: it separates hair from the backing so the key can resolve individual strands, and it overwhelms the green arriving from behind with white light.
Use a negative fill. Black cloth or black card just out of frame on either side of the subject absorbs bounced green before it reaches them. It costs almost nothing and works well in a small room where distance is not available.
How to set up a green screen has the full arrangement.
The wrong way to remove it
Almost every quick fix does the same thing: select the affected area and pull the saturation down.
It does remove the green. It also removes whatever colour was underneath, because saturation is not selective about hue. Brown hair with a green cast becomes grey hair. A red jumper with a green rim gets a grey rim. Skin under the chin goes ashen.
A slightly more careful version targets only the green range in a hue and saturation panel, which is better, and still wrong for the same reason: the hair contains genuine warmth as well as spill, and the tool cannot tell which greenish-brown belongs to which.
The right way
Think of spill as a push. The screen colour points in a particular direction in the colour plane. Spill moves the pixel some distance along that direction, and everything else about the pixel stays as it was.
To undo it: work out how far the pixel has been pushed along that direction, and subtract exactly that, leaving the pixel’s brightness untouched. The result lands where it would have been without the screen there.
Two things make this work where a saturation slider does not. First, it is directional: a colour that has no green in it is not moved at all, so skin, a red jumper and blue eyes come through unchanged. Second, keeping the brightness fixed is what preserves the sense of hair being hair, because a strand’s appearance is mostly about its brightness against what is behind it.
A good implementation gives you an amount control. A subject wearing olive genuinely contains green and will look drained at full suppression; a subject in white can take all of it.
Tell whether your tool is doing it properly
- Key a frame with obvious spill on the shoulders.
- Turn the spill suppression to full.
- Zoom in on a shoulder and on a patch of hair.
You should see: Hair that keeps its warmth and its texture, and a shoulder that looks like the fabric’s real colour.
If you do not: Grey, flat hair means the tool is desaturating. Check whether it has a separate despill control rather than a saturation one, and if not, do the key here instead and composite afterwards.
What suppression cannot do
Spill that is strong enough to blow out detail is gone. If the green light was bright enough that a section of hair is a flat pale green with no variation in it, there is no variation to recover: subtracting the hue gives you flat pale grey.
That is the case where you go back to the room rather than the software. Move the subject forward, add the back light, take the screen lights down a stop, and shoot it again. It will take less time than trying to rescue the frame you have.
Very fine detail against a bright screen suffers similarly. Chroma subsampling in the camera stores colour at half resolution, so a single strand of hair one pixel wide has its colour averaged with the screen beside it. This is why professional video formats avoid subsampling and why a RAW still keys better than a JPG of the same frame.
Light wrap, and why the fix can look wrong
Remove all the spill and something odd sometimes happens: the subject stops looking like it is in the new scene.
Real objects pick up colour from their surroundings. A person standing in front of a sunset has warm light on their shoulders and a rim of orange in their hair, and your eye reads that as belonging. Strip every trace of the environment and you get a subject that looks pasted on, perfectly clean and slightly unreal.
Compositors deal with this with light wrap: after removing the green, they add a little of the new background’s colour back along the edge of the subject, usually by blurring the background and letting it bleed a few pixels inward. It is the same effect the green screen was producing, in the colour of the place the subject is now supposed to be.
For a still image on a solid colour, none of this is needed and full suppression is the right answer. It becomes worth knowing when the cutout is going onto a photographic scene, and the giveaway is that the composite looks technically perfect and somehow fake. If you have that feeling, the edge is usually too clean rather than too dirty.
The related trick is to match the direction and colour of the light. A subject lit from the left, dropped onto a scene lit from the right, will never look right however good the key was, and no edge treatment fixes it.
Spill against halos
The two get confused, and they are different problems.
Spill is coloured light on the subject, present in the file before any software runs. Fixed as above.
A halo is a fringe of the old background left in the boundary pixels by the cutout process. It happens with every method, keys included, and the fix is colour decontamination: working out what the subject’s real colour was in those mixed pixels and replacing the mixture with it. White halo around a cutout covers that properly.
You can have both at once: a green rim that survives the key because it is on the subject, and a lighter fringe just outside it from mixed edge pixels. Deal with the spill first, because it is larger and it changes what the edge looks like.
When the subject contains green
Sometimes the problem is not spill at all. If the subject is wearing olive, holding a plant, or has a green logo on their shirt, a key set wide enough to remove the screen will start removing them too.
Three options, in order: shoot against blue instead, which removes the conflict entirely; narrow the key and accept a little screen left behind, then paint it out; or key normally and paint the affected part of the subject back by hand. Chroma key explained covers what tolerance is doing when you narrow it.
The free way
Take the cast off without taking the colour out
The spill control here subtracts the screen’s own hue from each pixel while keeping exactly the brightness the camera recorded, so hair keeps its shape and its warmth instead of going grey. It works on any backing colour, the amount is a slider, and the result is a transparent PNG at full resolution. Nothing is uploaded.
Questions
What is green screen spill?
Green light reflected off the screen and landing on the subject. It shows along the tops of shoulders, under the chin, around the edges of ears and through loose hair. The camera is not wrong: those pixels really are greenish.
How do I stop green spill?
Distance is the main fix. Move the subject a metre or more away from the screen, ideally two or three. Then avoid overlighting the screen, light the subject separately, and add a light behind and above the subject to give the hair its own edge.
Why does my hair look grey after removing spill?
Because the tool reduced saturation rather than removing the screen's hue. Desaturating takes out the green and the hair's own colour together. A proper spill suppression subtracts only the push in the screen's direction and leaves the brightness alone.
Can I fix spill after the shoot?
Yes, mostly. A good spill suppression recovers a lot, and for the small number of pixels it cannot, painting over them with a colour sampled from nearby works. What it cannot do is invent hair detail that the spill has washed out entirely.
Does a blue screen have less spill?
It has the same amount, but blue spill on skin often looks less wrong than green, because a slight blue cast reads as cool light while a slight green cast reads as illness. Blonde hair picks up green spill very visibly, so blue is often the better choice for it.
Is spill the same as a halo?
No. Spill is coloured light on the subject before any software runs. A halo is a fringe left by the cutout process itself, from the old background still being mixed into edge pixels. They can appear together and they have different fixes.