This may not be your solution because although it was 6 months ago and I forgot just exactly what I used but: I have a set of red oval grips off a 1955 Schwinn; (no photo currently) that were like yours. I tried cleaning them hoping something would improve, soaking scrubbing etc. that when wet looked good, but after dry didn't. I'd given up as soaking for a week or so, only made them worse once completely dry again so, they'd sat around for years. One day I went after them again. All or most of the discoloration is actually dried grease from usage, sweat, oil and the like from hands.
I think I used a good water solvent degreaser and although years before I am pretty sure I scrubbed them with plastic brush and soap, to no avail. This time I began breaking the old, dried grease and so just kept at it with a plastic brush, scrubbed and scrubbed and scrubbed until I got it all off.
They are not great as the red is faded for age, wear and UV exposure but, a major difference once I broke that spit off, they' red again. I can see the same crap on yours, but your rubber appears better than mine were at start otherwise, I wouldn't have replied.
So, there is hope; once U realize the rubber is not as bad as it looks, and U break the dried grease/oil off.