If you are referring to the color of the upper and lower, then there is little that can be done. The anodizing will continue to lighten slightly as time passes. The furniture will clean up nicely if you use some simple green, then re-oil with CLP. If you are referring to the steel parts, like the barrel, sight tower, carrier assembly, bolt catch, etc. then you can have them re-parked but I bet they will end up looking odd. Have you tried cleaning them with a solvent (acetone)? Maybe theres a bunch of LSA /CLP gunk built up on them over the years.
My 4 digit SP1 is like yours also. But I kind of like that light gray finish in contrast to the black furniture. Has a one of a kind appearence.