Why did Dr. Montessori choose the colors for the grammar boxes? Why don't they match the grammar symbols like they do in primary?
Because if they matched, it becomes a mindless matching activity.
So why use color at all? To aid the child in sorting the multitude of cards into the proper compartments. Also to show during transposing that the "verb" just became a "noun" - without the color, the incongruence wouldn't show up as clearly; the grammar boxes "game" would be less fun, fewer patterns would be noted, and the fullness of the activity would be lost.
All reasons and many more riches are described in The Advanced Montessori Method available free online via Google Books.
Please do not change what Maria Montessori developed - when we don't understand the true depth, we can inadvertently take away the true beauty in the experience.