There's been a recent thread on the list regarding a drastic color/contrast shift that finally got narrowed down to a LK or LLK cart replaced with the other ink. Somehow, the wrong cart made it into the keyed slot and until the user looked, he was baffled. He still is baffled why the cart fit, but found the problem, purged out the ink and is now printing fine.
If there is a drastic shift in color/contrast, it may be worth the look. In any case, I would probably pull the cart you just replaced and give it a shake, just in case!