Drawing board creation

hi! first of all this is a great site/forum and it had help me a lot

OK!on with the question

i am trying to make a drawing board(like colouring book) where when i click and MOVE THE MOUSE it colours the object(more like a brush and not as a paint bucket)

i would like to know how to colour objects individually(without coloring background) ?

change colours(actually no need i think i can figuer it out)but any help is appreciated?

and the most IMPORTANT is there any way to detect what percentage of object had been colored(like if 100% of object coloured something happens!)

thanks in advance:smirk: