My answer -- Four colors IS definitely enough -- was accepted.
I used "is" rather than "are" because it looked like the verb -- stači -- is singular. I figured that, since what I took to be the singular form was used, the actual subject of the sentence was "čtyři," not "barvy."
Did I just get lucky, or is my reasoning sound? :-)
UPDATE = = = I think I just got lucky... the form "stačí" is the same for both third person singular and plural.