I'm assuming what the problem is, is that students didn't keep the colors
clean the last time they were used. Why not, when you use these for a
project, show the kids the correct way to clean the cakes...and have them
do it.(I teach my students to clean the colors--of those and watercolor
boxes) by putting their brushes in water, swirling on top of the dirty
color, and repeating--until the colors are clean. They do this at their
tables, with the water containers.) I would suggest keeping one of the
gross ones to use as an example in the future. Before I start doing
watercolor with my 1-4 students, I always bring out the gross watercolor
box, open it, and ask them if they would be pleased if I gave them that
box to use. Then, I show them again how to keep the boxes clean, and
insist that at the end of class, that is how their boxes need to look.
Even the 1st graders get it. It's just a basic respect for materials.