How you can cease yellow discolouration on joystick?

That is for a pastel blue PowerA Xbox controller, I received it final Christmas and I don’t use it fairly often, however currently I’ve observed very slight discolouration on the joysticks and I don’t need it to worsen, I can’t determine easy methods to get the color again the identical as the remainder of my controller although – there’s some mud within the grooves I can’t get out too.

Im unsure what induced the yellowing, as my arms are undoubtedly not that soiled, and the controller doesn’t face the daylight when it’s on its stand in my room so idk, I wouldn’t have thought it will be sufficiently old or used sufficient for this to occur.

EDIT: tried hand sanitiser nevertheless it didn’t do a lot, didn’t even take away all of the mud :/