Why is my urine yellower after a triathelete workout?

Taking vitamins? If you mean by yellower, darker then it could be d/t dehydration as my colleague indicated. But if you mean brighter yellow it could be d/t vitamin water or any other vitamin intake used pre competition.
Urine pigmentation. Dark urine may be simple concentration from dehydration or more concerning like hemolysis from strenuous exercise. Drink moe water and get checked out.