I read this interesting article on Digital Arts yesterday: 10 Colour Secrets from Leading Illustrators. It's great getting some insight into how other illustrators work with colour. I might not be a "leading illustrator" myself, but I thought I'd share my own thoughts on choosing colours - maybe you'll find it helpful!
A limited palette I pretty much always work with a limited colour palette - anywhere from one to six colours. That Photoshop colour picker, loaded with squillions of options (or that huge tin of pencils, plethora of ink bottles, etc) is very tempting, but too much choice can be a pitfall. I think that limiting your palette helps with the logic, rhythm and flow, so the important elements stand out and the secondary elements recede and work to create depth and texture (or whatever they're there to do!). Limiting your colours also calls on your ingenuity to create something that's still dynamic or rich with detail and variation. I like to use halftones, pattern and negative space, rather than introduce more colour. I also love the charm of vintage illustrations where a limited palette was a practicality. I remember in some of my favourite picture books when I was little, you'd have the lovely glossy full-colour pages alternating with the pulpy, uncoated pages featuring one or two colour illustrations. I think I used to prefer those to the shiny colourfest! I think there's something really intriguing about what an artist does with line and tone when a full colour palette isn't an option.
Nothing's black and white Black isn't always black. I very rarely use pure black, preferring softer blacks, such as a desaturated dark blue, a warm brown-black from a yellow palette, or a dull, dark red. For something more subtle and muted, I also like to use less intense colours in place of black. As long as there's enough contrast, it can still work. White might mean 0% ink, but it's still a useful addition to your palette - especially if you're only working with one or two other colours. There are some really amazing illustrations around that use negative space and let the colour and texture of the paper/background do the talking.
Tonal values I usually try to work out my colour palette before I begin, but there are often changes as I go along. In trying to pick the right combo, I like to desaturate my palette and check the percentage of black for each colour. You can make lovely, muted illustrations using colours with a similar tonal value, but most times I prefer to have a range across the greyscale spectrum, to create enough contrast. I'm mindful of the saturation of each colour for the same reason.
Anyway, that's my five cents' worth!