I don't know if it would change the colour of his sperm but.. Has he had his testosterone levels checked? Low testosterone levels tend to lead to men not wanting intercourse.. There is also a chance maybe he's having a hard time getting it to stand up..
I know guys are usually embarrassed by that sort of thing, so he may just lie and say everything is fine even if it isn't.. Just letting you know, there could be other reasons for his odd behavior that doesn't mean he's cheating. Usually there's other signs if it's cheating, like change in attitude, work hours, hiding things (like his cell phone) and other stuff.
I will agree with the tips SM has provided.
I wouldn't jump to conclusions and ASSUME he is cheating.
There could be a number of other reasons than cheating.
HI there and welcome. how's the rest of your relationship? You two getting along well? Clearly you aren't communicating too well or otherwise you'd broach the subject of the less frequent sex by asking him. You could just say "hey, we used to have sex more and I miss it. Everything okay?"
good luck