How to increase your melanin without exposure to the sun?

Not possible: Anyone telling you otherwise is scamming you. There are various spray-ons to create a fake suntan; this is your choice. Skin color has very little to do with whether one looks good / is attractive to others -- your level of physical fitness totally overrides this (you completely control this), as well as how you carry yourself and your interpersonal skills (can be learned / taught). Good luck.