I do that too. But, for me, it seems like I go through phases. I currently have a good stash of blues and pinks. I've never been a "pink" person but lately it seems that's all I buy. I have a very small stash of yellows and greens. I love purple but have a very small stash of that too.
I force myself to step out of the box when I look for more fabric so my colors are getting there.
Don't know why I do this.....weird.....