A limited palette will reign in your work and if done well it will get a balanced and harmonic feel. That said, even if you work with a limited range of
Hues (think of these as colour tubes), you may often want to work with a wider range of
Values (i.e. mix more or less white/black into the base colour).
When painting on the computer you will get very far (also without extensive colour theory knowledge) with the following simple setup:
1) Use the paintbrush with a 30-50% opacity
2) Make sure you know the keyboard shortcut for "pick color from canvas" (ctrl+click is normally used)
3) Set up a keyboard shortcut for varying Value (nothing is set by default)
4) Pick a few (usually 3) colours (Hues) from your palette that you want to use in your painting. Dabble these down where appropriate.
Henceforth, hide the colour requester. Just use the colour picker to pick up the colour you want directly from the canvas, and adjust its value as needed with the keyboard shortcut. Only paint with these colours. The interesting feature with the method is that since you use low-opacity for your paintbrush you will get mixing of colours - and you can pick up the mix. The result will be that you can in principle deviate from your main 3 colours somewhat. This will generally make it much easier to reign in your colour use than if you were picking colours from the wheel every time you wanted to change it (easier the closer together your 3 original colours were placed on the colour wheel). You can always add extra colours in the end to spice/highlight things but that's just for final touches. Maybe worth a try to experiment.
Of course, there are many ways to go about it and eventually you will want to read some colour theory to at least have a feel for why certain colours look better together than others, how to colour shadows etc.
Also, MyPaint has full palette gamut masking features as of version 1.1 (as far as I know it's the only graphics program anywhere with this feature, go OSS!) - this means you can explicitly mask out your colour wheel to only allow you to use certain parts of it. Worth checking out:
.
Griatch