First off, layers are your friends; combined layers even moreso.
By adding a layer over the top of the picture, you can modify the colours of the picture underneath. Each layer has a variety of properties you can apply to it that affects the underlying image in different ways.
A Normal layer just overlays with flat colour.
A Colour layer changes the hue to whatever you paint on it, while maintain the tone from the layers beneath.
Lighten layers change any colour
darker than the colour you're painting
to that colour, while leaving any lighter colour untouched; a Darken layer does the opposite. And of course, by altering the opacity of a layer, you can increase and lessen the effect it has.
Try taking an original image, applying a couple of areas of flat colour on a layer over the top, and cycling through the different layer types, to see how each type affects the colours. Then try overlaying two layers with different properties and see what that does. You'll soon get a good feel for what they're capable of.
<edit> As for colours, how's your knowledge of additive colour theory (that's how colours work in light, rather than subtractive, which is how they work in pigment)? Bearing in mind you're working with RGB on screen, you can simply play with those values to get the colour you're after. Say I'm after a light orange, I'll put the red up high, the green about half-way to get the hue (red and green make yellow, less green takes it toward the orange), then add blue to lighten it.
And yeah, I'm colourblind too.