How do you select select your colours? Please describe in detail. Any detail is important to understand your issues.
I go onto the the right side bar where it says "Colours" :S
this is what it looks like
I select a colour from the side then try to paint with it and it automatically changes to white
Try changing the stroke colour instead of the fill colour.
At least I assume that this is what is causing your trouble.
it works! thanks so much!