Don't change you oil based on looks.... just because it's dark doesn't mean it's bad. You can actually cause problems with some oil formulations by changing too frequently... and you may just be wasting money you could otherwise be saving for upgrades.
Getting your oil tested will let you know just how little you really do know bout oil and also what the lab finds in it will let you know (with scary accuracy) just what is about to fail in the engine.
http://www.blackstone-labs.com/