I was thinking of making up a batch of different royal icing flowers, just to practice and play, so i was thinking of doing it all white, and then painting with food coloring or petal dusts later when i needed them (i've never used petal dusts) will this work, or do i have to tint the frosting first in order to have them look decent?

I use powders with lemon extract. It dries quickly and it gives good color. Vodka works well too if you don't have lemon extract.