Things depend on what kind of style your going for with the colors?
Do you want something painterly? Cel shaded? Something like a comic book where you can still see the lines or a semi realistic render?
These have somewhat different approaches but also kinda similar.
But some general things to keep in mind would be as follows.
1. Knowing your light source direction. Since this takes place outside, you can use a simple 3 point light setup.
Your Sun is coming from the top left.
Blue Sky light from above
Sandy light browns from the bottom right side
What you would do is apply these colors/lights to the parts of the forms that face each light.
(Hope that made sense)
I did a quick and very simple version of what I mean using basic shapes.
When I say apply 1 light at a time, you can have separate layers per light as well. I like to use the overlay blend mode as well as soft light and hard light at times. You can play with others like lighten. Just use what looks cool.
You can use the basic shapes I laid out if you want.
Don't forget your shadow layer as well. Use a nice cool color like a purple, or some variation of a blue color and set that to multiply. Try using something that isn't to saturated.
Something in this range.
Focus on the drawing first and get that looking good.
Then move on to your flats on anther layer
Shadows on multiply
Sky light, and Sun Light on overlay
Bounce light works nice on normal. Lightly press down with your pen.
Extra highlights layer and or rim light can be added after
The sand storm can come last probably
If your doing a Cel shaded look
Keep your line art and flats layer. What you'll want is a nice gradient for your colors
You'll keep your shadow layer and highlights layer as well.
Anyway I know this was super long. Sorry for that. Hope it helps though.