Melatonin and Melanin are closely related chemicals that are used all over the body and are synthesized in the body through related mechanisms and pathways. Melatonin is used in the sleep-wake cycle and melanin is used by the skin to darken it and protect from solar radiation. When you need to sleep and when you need solar protection tend to be inversely related. The interaction between these processes is still being studied and is likely to involve many other things currently not under study.
In general, unless you are under the close supervision of your doctor, I would HIGHLY AVOID injecting peptides or really any other thing into you. In relation to just melanin and melatonin, the interactions are complicated and not well understood. We likely do not have the full picture of what melanin is doing in your body and there could be big problems. There also could be no problems at all. That said, I would not want to be the test cases for a M&M conference on this.
I do not think that people on /r/peptides are doing anything but unnecessarily putting their health at risk and (possibly) doing harm to their families, friends, and other relationships. As we know so little about this area of research, the risks are not well constrained and could result in death or worse. When it comes to health and wellness, especially biochemically, it is much better to leave it to ethical and well trained medical researchers that know what to look out for and how to help when things go bad. Auto-biochem hacking is a hard pass from me for now.