Can the Oxford Comma be used with a list of adjectives of the same type?
Yeah, it's fine.
The Oxford Comma: Hart’s Rules
...
Examples of the serial comma are:
- mad, bad, and dangerous to know
http://blog.oxforddictionaries.com/2011/06/oxford-comma/
There is some ambiguity in the case of your tulip example because it's not clear whether each tulip had three colours or whether you had a mixture of single-coloured tulips, but I'm guessing that's beside the point.