My first thought was black. It's my default. I think it's genetic.
But here I may try a white and black fleck paint. I used it a while back. For an ashtray, it gives some texture, and would look the same color as ashes. If it was dusty, you wouldn't notice. If it had ashes on it, you wouldn't notice. A bit fancy for a stinky ashtray, but it may be a good long term, low maintenance solution. Especially if you have a significant other who may complain.