IMHO, limited to two distinct colors shading to their lighter ones is ok as far as the speed of shading is constant for both colors and there is no common color between them so the player don't get confused about what color/state is being showed. Also, the shading animation may take shorter time than the amount of time needed to change to the other color, being this the case you should keep the lighter color in a static fashion or maybe loop the animation (putting back the darker tone of that color and shading again or maybe shading back to the darker tone) but i am not sure if the latter follows the rules of the jam :/.
Greets.