Hugh, as you know I'd normally go for blue.

In a nutshell, blue has greater colour difference from most of the likely foreground objects one is likely to need a key from. And fg/bg colour difference is what one is really after.

Blue might be worse in some situations: - blue fg objects (duh) - dark/unlit fg where there IS no fg colour, and one is therefore totally reliant on the bg colour for an edge

IMHO, the fact that the blue colour channel (on film and HD) tends to be the noisiest one is a red herring, for the reasons outlined above - an opacity matte is based on colour DIFFERENCE, not just the backing colour.

Green has its own problems: - less colour difference, not that far from skin, sand etc. Green spill is therefore quite hard to get rid of without messing with the colour saturation of the fg. - green can end up horribly overexposed in sunlight exterior situations

It's rare to see a really great blue OR greenscreen element nowadays. If a studio bluescreen is done really well (great backing material, heavy blue filtration on the screen lights, totally even exposure) then you can get a perfect key with one button push.

Preferred keyer: Keylight.

As a generalised answer, green.

• It's most luminescent. Our eyes care most about it so capture methods aim to capture green cleanly. With film it's green is usually less grainy than blue, and digital cameras tend to hide compression artefacts/noise in the blue channel

• Bayer patterns favour green pixels over red/blue (RGGB), so it theoretically has a better resolution (quite how much this matters is debatable, I suppose)

• Blue clothing (jeans, navy-blue suits) is typically more common than green clothing.

• Totally unfounded, but I've always found green screens easier to key.

Of the prebuilt keyers I've run into (mainly Keylight and Primatte), I've used Keylight the most.

I don't really like how Primatte works, it seems excessively complicated, and always felt like the various selections were fighting against each other. I may be slightly biased against it, as the OFX version was buggy with the build of Shake I was using (it would work fine in the viewer, then when I connected any node, different output. When I rendered, different output..)

Keylight seems much simpler, you set the with screen colour and play with the magic "FG bias" colour and generally get a decent starting key (even with hideously under exposed and uneven bluescreen I've gotten remarkably good keys just by playing with the FG bias..)

Another thought - I'd love to try using a CYAN screen. It's just halfway round the colour wheel between blue and green, and in that sense, is just as much a saturated colour as either blue or green. And, in a situation (unlit/dark fg) where the opacity matte depends entirely on the backing colour, with cyan you get the sum of two colour channels (half the noise?).

On film, I think cyan is kind of out of gamut? But for HD, maybe worth trying?

Certainly, you rarely get fg objects that are cyan...

Please tell me why I am an idiot.