The dichroic filter in the heads are pretty stable, though their values may differ from model to model. I have two Chromega D color heads. One came with the enlarger, and the other I picked up at a fire sale. At first they did not match up at all, and I was scratching my head trying to figure out why. So I lit them up side by side looking for color temperature differences. It was obvious upon comparing them side by side, that one was very yellowed with age. Opening up the light mixing chamber revealed that the reflective lining had indeed yellowed after years of exposure to the high intensity halogen lamp. After refitting the yellowed lightbox with new reflectors, they matched up pretty closely. I'll bet that if I changed the lamps and the IR filters out with brand new parts they'd be even closer. As it is, I don't bother. It's not like I'm going to be swapping heads for no reason other than to say I did it.