Which should I apply first between Retinol and discoloration serum containing tranexamic acid and kojic acid?

Personally I would say not to use these on the same night. I'd rotate every other night. The other choice would be to apply Retinol to dry skin. Then wait about 15 minutes. Dampen the skin, and then apply the Discoloration Defense over that. "This answer has been solicited without seeing this patient and cannot be held as true medical advice, but only opinion. Seek in-person treatment with a trained medical professional for appropriate care."