I've spent most of today editing together a short video trailer for a project that I won't mention specifically on this blog, because for various reasons it makes sense to keep the two things separate, but many of you will know what I'm talking about.

The video was filmed in a makeshift green screen (or chroma key) studio I've recently constructed in my spare bedroom, using a combination of proper kit (e.g. a large piece of specially-dyed material to use as the backdrop) and cheap, make-do substitutes (e.g. the halogen light from B&Q I mentioned in the last post).

Final Cut Pro has a bunch of tools you can use to remove all the green from your footage so that the background you really want behind your image shows through.

Now this is all well and good in principle, except that I'm colourblind, and green is one of my problem colours. Don't get me wrong, when I look at this 12' x 10' green screen, I do see a big expanse of a colour I understand to be green, but I imagine it just doesn't look as fundamentally, undeniably green as it would to you.

It's obviously difficult for me to describe exactly how what I see differs from what someone with perfect colour vision sees. It's a bit like asking someone with no sense of smell exactly what it is they can't smell, but I'm not particularly aware of having a diminished colour spectrum. I look around and the world looks like a fairly colourful place, but I guess there are certain colours that look similar to me, that probably wouldn't look similar to you.

Of course, there must be colours that look very similar to you too, it's just that in your case it's because they are similar. For me it might be blue and purple, or green and brown. In my primary school drawings my skies were often purple and my trees had brown leaves and green trunks.

So there is some irony to be found in me sitting in front of a computer trying to adjust an image based on the inclusion or exclusion of subtle shades of green. I think I can see well enough to do a reasonable job, but then it could be a complete mess and I would never know until I showed it to someone else.

But there is a precendent in my family for this sort of thing. My grandfather, from whom I believe I inherited my underperforming rods and cones, seems to have gone out of his way to seek out jobs for which you would have thought colourblindness would have been a major impediment.

During World War II he was a paratrooper. Now maybe they were less fussy back then, or weren't even really aware of the concept of colourblindness, but these days that would rule you right out, because you might not be able to tell the difference between the green (“jump!”) and red (“don't jump yet”) signal lights.

(This is a reasonable concern because from personal experience I know that red and green traffic lights aren't as startlingly different to me as I hope they are to most people, but I get along just fine by knowing that the one at the top means slow down and the one at the bottom means keep going).

And for several years while I was growing up, my grandfather worked for a large newspaper printers, and was in charge of checking the colour balance of the very first Sunday supplements as they rolled off the press. And as far as I know he wasn't fired.

But this whole thing has got me thinking — I wonder if I can get a guide dog for being colourblind? I've heard it said that dogs are colourblind themselves, so maybe that's a non-starter, but the idea of having a dog in the passenger seat barking once for a red light, twice for amber, and three times for green does have a certain comic appeal.