Rae, I had heard this somewhere before too (not from Doctor Who) and I thought it was right on the money. In fact this conversation has come up recently -- I think it was from a post by Fruitful? Anyway, I must have articulated this at some point in the past, and she remembered, and then paraphrased me in her recent post. What I had said was actually more specific: if you pair black bottoms with a bright top, the bottoms look boring and the top looks cheap. To my eye, the same doesn't seem to be true for bright bottoms and black top, although I don't have a clue why that would be so.
Sorry to anybody who likes black bottoms paired with bright tops. I don't think it works, ever. (Sorry Angie). Or, well, it doesn't look terrible, but it doesn't look particularly good, either. Could be that I'm wrong, or narrow-minded, but if so I'm not the only one -- certainly the writers of Doctor Who, at least, feel the same way.
You know, this does remind me that colour choice is so, so personal. For example, Sally did a post recently where she asked about people's "fashion powers" -- i.e. your particular strengths when it comes to fashion choices. She went on to say that her special power was a facility with colour combinations, which made me spit my coffee -- because I feel that colour choice is the LEAST distinguished aspect of her blog outfits. She does flattering silhouettes really well, but then wears these really jarring (to my eye) colour combinations that make me think of clowns or flea markets. But clearly she thinks it looks great, and probably lots of other people do too.
All this is by way of saying that it's really hard to make rules when it comes to aesthetics, because qualitative elements like elegance, harmony and beauty are always subjective.