I think that comma was correct. Try saying it (the whole sentence I mean, not just "big red bus". He wasn't an adjective phrase describing the bus but an example of a list of adjectives that could form an adjective phrase. You instinctively put pauses between "big" and "red" because you are talking about it as a list. When you are talking about lists you put commas between the elements, and when you take pauses you put commas. So there are at least two reasons why a comma belongs there even though when using adjectives naturally you don't put a comma.