Yes, they're green

I took the colour-test today (because I was watching my tweets and it's sputum day which means lots of incubations) and scored pretty badly. Here's my chart:


Not that surprising as I'm red/green colour-blind (not sure that I should really have that massive peak in blue though :s ). Or to be more precise I have deuteranomaly, which is mild deficiency in distinguishing certain shades of green. I've known about it for years. When I was at school I was taken to the clinic for some sort of routine check-up (I think I must have been 6 or 7) and they checked my hearing with cool retro headphones that looked pre-WWII and gave me the Ishihara test. It's those circles filled with various sizes and shades of dot.

"What number do you see?"

"21, no wait 27"

"Is the woman wearing a hat?"

"What woman?"

"Can you see a teapot or a mug?"

"Mug"

I felt like a mug too. And who was that mysterious woman in the dots?

After that came another test that has a bit more finesse than the Ishihara (I haven't been able to track down what it was, Colblindor's blog offers some alternatives though) where you compare four outer dots of colour to the middle one and say which is the closest. On this one I wasn't completely colour blind, just slightly. There's something a bit like those tests in the Exploratorium here in SF (which is really good fun, I've been twice) and my parents (who are visiting, hello Mum! Hi Dad!) and I all picked different shades to match the central dot. They both picked ones that were on the far extreme from mine, but interestingly theirs were different to each other.

The doctor at the clinic then handed me a badly-photocopied, poorly-typed list of jobs that I wouldn't be allowed to do. I think I remember it being an A4 sheet with about one and half columns of jobs which included working in a paint factory, being a jet pilot and being an electrician. I had pretty much decided that I wanted to be a marine biologist by then, preferably one that worked on 'things that live under rocks by the beach' (it's gone off track slightly) so wasn't really bothered (in fact I felt a bit special, like I'd passed the entrance exam to a members only eye club), but that's probably not great for someone who has their heart set on being an electrician.

But what does being colour-blind actually mean? First off I can see colours; red, green, blue, dusky peach, rosy nipple, slightly off milk and magnolia. When someone finds out they invariably point at a tree and say "what colour is that?" and I say "green", knowing of course that trees are green and they are stupid for pointing at something that is quintessentially green. They're marginally more impressive if they point at a deciduous in autumn. But still, it won't fool me. I also have no trouble telling apart the live, earth and neutral wires in a plug.

What does fool me are weird shades of green; dark greens at dusk can look dark grey or black and I have a nice brown checked shirt which is apparently green. Not proper green, rubbishy green that looks a bit like brown. Another thing that I find difficult is telling apart horrible green from horrible orange. Wikipedia helpfully points towards an example of this at the New York times. Though I can see the difference in the two hues in the key, on the map I can't distinguish the range of the whitebark pine forest from the high-risk zone of pine beetle infestation. Perhaps cartographer and fumigator should be added to that list.

There are two types of cell that sit in your retina and respond to light, rods and cones. Cones deal with colour and come in three flavours, blue, green and red (so far, so GCSE integrated science). My reds and blues (S and L cones for short and long wavelengths of light) are fine, but the greens or M cones (for medium wavelengths) . Take a look at this picture, if I had deuteranopia, which is a complete lack of M cones, then I wouldn't be able to see any of the colours under the green line on the graph (assuming they're not covered by one of the other two cones). Instead I'm deuteranomalous, which means the green line would be shifted to the right so that it almost, but not quite, covers the same area as the red one. My green cones are doing their best impression of red cones, hence it can be hard to distinguish between some shades of red and green because both my M cones and L cones are getting excited. This also explains why some shades of green are difficult to see in their own right because they no longer excite the M cones at all (they've moved on, that green was old news, red is the new black).

What's really interesting though is that it might be possible to have all four kinds of cone. Humans are trichromatic, they have three types of cone cell, but some women could have four: the blue one, the red one, the normal green one and the abnormal green one and would therefore be tetrachromatic. And if they had a son he'd be colour-blind (Mum's feeling all special now too - it's not well researched though and wouldn't be all women with colour-blind children, sorry). If you're feeling really scientific, evolution of colour vision and the opsin genes is fascinating (try this one for starters) and gets creationists' knickers in a twist too.

So, give me a paint brush and I might paint something with slightly more intense greens than you would (it's been hypothesised that Van Gogh was blue-yellow colour-blind, check out his Sunflowers or Starry Night but keep in mind this). Shine a light at me from the end of a corridor in the dark and I might not be able to tell whether it's green or not (this is the basis of the Farnsworth Lantern Test which colour-blind sailors have to pass if they're going to act as lookout, it separates the mild from the severe. For scientific cruises you have to pass a sea-going medical and mine was stamped with "no lookout duty" because they didn't have a lantern and I've never been very observant of icebergs), but really post-boxes are quite definitely red.