OK so the cup doesn't have to be red, but the song makes it more fun.
Are regular 16oz plastic cups big enough for up-potting tomatoes? I always drill a hole in the bottom (or burn one with a lighter but BE CAREFUL) to allow for drainage, AND it doesn't make me feel so bad when I give tomato plants away and don't get the "pots" back.
And is clear plastic be bad? Is the algae that grows inside harmful? It's fun monitoring root growth, but should I switch from clear to colored?