If there's a tile store near you, get Laticrete caulk
Then make sure after the curing process to rub Nufinish on it. This will keep the mold from growing.
Re-apply Nufinish every 3-4 months.
If the caulk has become discolored, use the Nufinish like a cleaner and it will look good as new
Works like a charm.
This was the recommendation I got from Bath Fitters.
At first I didn't listen to them, but after the first silicone needed to be replaced, I took their advice and no more moldy caulk