With the contours traced, I can Hide the "Tracing Image pod" and fill every single part with a different grey (see color pod)
that I painted with the ink pen.
I grouped those objects in a folder named "Masques_1".
From now on, I am able to isolate a single objet (like "Mask_Flower_1) by selecting that layer and calling => "Edit - Select Layer Contents".
Make that layer (mask) invisible, create a new layer to put your colors in, and paint within the borders.
From now on, I will never paint an unwanted color from "Flower_1" on "Flower_2"... And I can use a large (and very very large to...) brush
to build up my flower colors.
Does it make sens?