My Bailey Senator is now six years old and the sealant round the shower tray is yellowing. I'm sure this has happened to other showers so please tell me what I can do.
Remove and replace. The sealant has obviously deteriorated due to environmental conditions (minerals in the water, UV, ozone, reaction with other polymers, etc.), so replacement is going to be your only option if its appearance bothers you.
Adding to Lutz's comments, if you do this ensure you remove all old sealant as new sealant will not stick to old and can allow leaks. There is a solvent to do this but check with your dealer it is safe to use on the tray and wall board. At home I have found that a cleaner called Oz cleaner removes stains off mastic that others won't touch. It is expensive and check it is safe on the tray.
We have had to do ours because of poor adhesion.I cut it off with a stanley knife,being very careful.Removed the screws around the tray to the wall and cleaned out and dried.There was a lot of slimy residue in the gap.I went round the bit to be sealed with a green scourer to slightly roughen the plastic,cleaned it with UPVC cleaner then put sealant between the tray and walls.Refix the screws which will sqeeze out the silicon then smooth it off removing the excess.I found the corners didnt match the profile well so built up a slight rim there.I also found it leaked past the door bar(we have an Indiana) so did the corners in clear silicon which has kept the bedroom floor pretty dry