it is normal that your fat pad of penis is darker tone than that of rest of your body.
There is no relation of penis fat pad colour with prostate cancer
Your third question is not very clear to me. Prostate cancer is a disease of elderly population, symptoms are often vague, merges with symptoms of benign prostate enlagement. Only way of suspicion of prostate cancer is rising PSA value this issue can be consulted with your urologist
. penile pad of fat swelling may be related to some disease but as you have mentioned it is not hard so any fibrotic penile disease are ruled out. Proper clinical examination may be necessary. Take care.