I am not sure why but my penis is two distinctly different colors... the bottom of my shaft is dark brown while the top half is tan... is this normal?

Sounds ok: One can ask one's primary care doctor to look, if one is worried. Having a darker bottom and lighter top is normal if it looks symmetrical and has been that way since puberty started (but with more noticeable coloring over the past few years). Doctors check the penis at annual check-ups, but rarely say anything if such coloring is there, as it is considered normal. So, point it out to the doctor.