Neutrals or blenders. And if I plan ahead at all, I'll take a quick look at my stash and see if there are any obvious gaps in my colors. For six months or so, I was looking for browns as I was low on them.
My main addiction seems to be neutrals, though. For a while, I didn't seem to have them when I needed them, so started picking them up when I saw ones I liked. Now I'm planning a quilt made of all neutrals. Can't ever get enough!