Hi A soap/ body shower gel has too little time to work on the skin to remove  a tan. In fact, using a loofah or scrubber during a shower can actually increase skin pigmentation called macular amyloidosis or frictional hyperpigmentation. Please meet a dermatologist so they can advise you on specific creams to use at bed time overnight to help.  Darkening of skin over neck and over body could also indicate underlying vitamin deficiency or metabolic issues such as hyper insulinemia.