How to clean bottom of 2004 Kohler tub?

I had a top-of-the-line (but otherwise ordinary white) Kohler cast-iron enameled bathtub installed in 2004 as part of a complete bathroom remodel in a small apartment attached to my home. This tub bottom has some sort of special surface pattern (slightly rubberized, by its feel) that is clearly intended to be "non-skid." I instructed my tenant (nice, responsible, I should add, though perhaps not the world's best cleaner) to clean it only with a mild soap and gentle sponge. Now the pattern on the tub bottom is rather discolored or just somewhat worn off--I can't quite tell which. The area on the tub end nearest the drain is perfect, the part in the middle of the tub is yellowish in places, sort of streaked in others. A cleaning lady who came after the tenant left tried a number of cleaning products and it made no difference in this discolored pattern at all. QUESTION: Any idea if the appearance of the tub bottom can be improved? Is this sort of wear "normal"? I have another tenant who ...  more