Great question. While I'm actually on the hunt for the "perfect" black silk sleeveless top, I am not going to say black. Because so many of my top layers (cardis, jackets) are black already. So I want contrast with the blouse or shirt underneath.
Cream or white is definitely high on the list, but nothing beats a really flattering pop of color. It's just too hard to pick only one. I'd get too bored with just one color. Actually I'd get bored with tops only in solid colors too. Patterns are too much fun.
So now you know why I own so many blouses.... and have been adding fast & furiously lately, since it seems Fall is the best time to find the kind I like (silky, sleeveless or short sleeve, rich colors). Jeans+black jacket or cardi+interesting shoes+interesting blouse in interesting color... easy formula!