Before applying anything, shower and gently scrub problem areas using washcloth or loofa. Use a moisturizing soap, maybe with olive oil in it, or Dove. Rinse very well, rubbing with your hands or a clean washcloth to remove any soap residue. After drying off, apply moisturizer to elbows, knees, ankles and feet and the area on the shoulders where you are having the problem. Then after moisturizer dries apply the tanning lotion evenly. This should help a lot.
This can be due to tinea versicolor. Tinea versicolor is caused by a fungus that lives in the skin of almost all adults. Factors that can cause the fungus to become more visible include high humidity and immune or hormone abnormalities. Treatment involves topical and oral antifungal medicines.
Please get it checked by a dermatologist to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.