DS5 was once doing a worksheet at home that said something along the lines of, "Color in the figures that have 3 sides". He made one little stabbing sort of line of color in each of the 3 sided figures and was done in 10 seconds. I asked him shouldn't he color them in, and he replied that the instructions read to color inside the figures, ie to "color in". He hates coloring.
The anecdote isn't a help to your DSs situation as the instructions probably don't read that way -- but is to say that while there are 4 kids being admonished for coloring too much there could be just as many being admonished for coloring outside the lines, or not enough. Or perhaps that will be next year and your DS will be rewarded for accuracy. One can hope.
I would re-approach the teacher and ask to brainstorm together about solutions to this loss of playtime, as your child is feeling discouraged. Especially if he did have ADD then time management is something that needs to be explicitly taught. The intent of the playtime reward is to teach speediness. But he's had nearly a year to have that method work and, as wonderful a method as that is, it just hasn't worked for him. Instead he just feels down about himself. So now at this point it's clear he needs some other method. If it's loss of playtime due to slow worksheet completion that's the issue, try to stick to it only and not get into the differentiation issue or him being slow to put away his coat etc, on that same day.
As one possibility to start the list of possible solutions, perhaps the teacher could alot him (or you could provide) a 5 or 10 minute (silent) timer that he turns on when he starts a coloring worksheet. That would make it very official when he is to stop, perfectly colored or not, absolving him of some of the responsibility of knowing how perfect to get about it.
Hope you find something that works. Perhaps read a book on managing ADD for more ideas of what to suggest to the teacher, even if you don't feel he has it. She might be most receptive if proffered solutions are in line with her pet diagnosis.
