Odd Exhaust Port Coloring

Getting ready to build a new set of headers, so removed the old ones today. Exhaust ports are a little curious-looking, the front port on both sides of the motor are quite dark, and the other 3 are a very light whitish-gray (see pics). Maybe an anomoly of throttle-stop racing? (My Dedenbear throttle stop has the secondary blades almost completely closed when on the stop). Both pictures are of the right side of the motor, the left side is identical. Any thoughts, opinions, or similar experiences?

2nd pic.

The grey ones are normal. The black would usually indicate oil in the exhaust side. Likely from a guide leak.

Thanks Al, I didn't even think about a leaking valve seal, they don't have much time on them. Guess I'll be replacing them.

Could it be fuel distribution?? I have always seen oil streaks with a leaky stem seal.

My first thought was fuel, the residue is not "greasy" like you usually see with oil in a combustion chamber. I think Al is probably right tho, for those front cylinders to be that much different than the others as far as fuel distribution, would really be hard to explain. I will comment more after I get into it--

This is typical of manifolds that have poor fuel distribution.The 4 corners tend to run richer and even the plugs show more color at the corners.We seen this on Indy manifolds and had to cut back the runners to equalize the fuel all cylinders.