If they turned tan or as you say a yellow color, then someone in the house is very likely a smoker?
Seen this many times and its always been where someone smokes. Side note, maybe the buttons turning brown is trying to teach a lesson?
I saw one that turned a dark brown and they stated they actually had a house fire!
Cleaning is possible I'm sure, but presumably would it also remove the numbers, lettering, etc. from the buttons?