Firstly, this isn't exactly a step-by-step tutorial, I'm not going to slow down to explain every technique. It's just a write-up of some of the process that went into this piece.
One feature that makes digital art unique is the ability to non-destructively try out many different variations, either through layers, history, or saving alternative files. So sometimes when I feel a picture can go in a few different directions, I do tests like these, to explore which direction I like best. WARNING! It can drive you crazy if you're indecisive or prone to procrastination.
Color can be very important in creating mood. To match the nostalgic subject matter, I wanted to use a vintage, slightly faded/yellowed look. One way of achieving this would be to restrict myself to particular colors, sort of to emulate a gouache 1950s sort of look - or analogous to using a particular kind of film. Another method is to use 'true' colors first, then altering them "in post" using Photoshop, analogous to color grading in movies, or hipstamatic filters. I went for the second approach.
In fact I go through this 'post process' in pretty much every picture I do, for two reasons. The first is that whether painting or alpha-masking, Photoshop tends to blend colors linearly, which makes the blends look a bit gray and flat. I think the same is actually true of digital cameras - they capture 'true' colors without the 'personality' that analogue photography tends to have. Playing with contrast, color balance, levels etc is a way of giving more life to the color palette. There are other ways to compensate for this, but this is how I do it.
The second reason is sometimes you can find surprising or unusual color combinations that you wouldn't have found intentionally. Traditional painters might balk at this, as a lack of planning or knowledge of color theory. Fortunately, this is not traditional painting
So, the first picture is the original base colors. The other five use various methods, like color balance (try shifting highlights towards cyan, shadows towards red, for example), using a hue/sat/lum adjustment layer using 'colorize', then set to 70% opacity, using an orange layer set to soft light blend mode. The possibilities are limitless. You could even print it out and scan it back in to get printing artifacts, then blend it with the original to soften the effect.
There are things I like about each of these variations, but eventually I went in the direction of #5. In the final version there are tweaks to some of the colors especially the background, and a texture overlay to give it a papery feel (in retrospect I wish the texture didn't have so much jpg artifacting).
For those who are curious, it's not vector art. I use Photoshop's polygonal lasso tool. You can find many other pieces using the same technique in my gallery. It's a very obsessive compulsive way of working.
Checking out the finished work here: [link]