My experience with using light color as an information source says that's a bad idea. You can only reliably communicate maybe about eight or ten states with one

My experience with using light color as an information source says that's a bad idea. You can only reliably communicate maybe about eight or ten states with one light before you start getting into "is this yellow or orange, I can't decide". So maybe "you'll just miss the next train if you leave right now" and "you'll catch it if you leave right now" could be augmented with a third state, "hey you'd better leave RIGHT NOW if you wanna catch the train". But a slow glide from red to green is going to have too many interim states.