When DPS goes up the number turns green while it goes up. While DPS goes down it turns red. What does it mean when the DPS number turns blue or yellow?

I notice it flashing yellow or blue ever so often, however it happens so quickly that I can't figure out what is actually happening. I hope someone can clear up this mystery for me.

I've tried to screenshot it but it happens too fast. I don't think I've ever seen it on individual Crusader DPS, only on the overall DPS on the top of the screen.

I actually noticed it quite a long time ago but hadn't given it much thought.