The fact that colour is a slippery subject is powerfully illustrated by my favourite optical illusion. Look at this:

Squares A and B are the same shade of grey. It's so hard to believe that you might need to see the proof to be convinced. 

Chromostereopsis is a similarly disarming effect that you may have noticed on maps with bright spectrum colour bars. Most people perceive blue and red on different depth planes, so the pseudo-3D effect can work in your favour and make the map 'pop' (This is not a good reason to use a spectrum colour bar, however... more on this next time). I notice that at least one set designer knows about the effect, making William Shatner pop on the TV show Have I Got News For You:

Color is a fun way to test your colour intuition. The game starts easy, but is very hard by the end as you simulatneously match colour tetrads. The first time I played I managed 9.8, which I am not-very-secretly quite pleased about. But I haven't been able to repeat the performance.

X-Rite's Online Color Challenge is also tough. You have to sort the very subtle colours into order. It takes a while to play but is definitely worth it. If your job depends on spotting subtle effects in images (like seismic data, for example) then stand by to learn something about your detection system. 

Color blindness will change how these games work, of course, and should change how we make maps, figures, and slides. Since up to about 5% of a large audience might be colour blind, you might want to think about how your presentations look to them. You can easily check with Vischeck and correct images for colourblind people with the Daltonizer. They can still be beautiful, but you can avoid certain colour combinations and reach a wider audience.

I have lots more links about colour to share in the next post, including some required reading from Rob Simmon and Matteo Niccoli, among others. In the meantime, have you come across any handy colour tools, or has colour ever caught you out? Let us know in the comments.

The image of William Shatner is copyright and courtesy of Hat Trick Productions Ltd, London, UK, and used with permission.