When my DD was this age, I gave her only 2 colours....red and white...or blue and white work well...then you get a prettier result! Avoiding any green or black helps....also, I gave mine feathers and sequins and she put those on too.
But really...you need to let her go a bit mad and the result is then truly hers.
put masking tape around the edges going about an inch into the front so you're left with a neat white "frame" at the edge of the painting. If you choose your colours well and don't give her anything that ads up to brown it'll look nice
so don't mix colours which between them contain yellow blue and red, so for example no green plus orange!
good combos: blue red and white blue yellow and green red yellow and orange purple white and pink etc