The bottom of my mouth is a weird purple color what does that mean?

It Depends: Since we do not know your ethnic background, it could be a number of issues. If you are black, or of mediterrainin desent it might be normal pigmintation. If you are fairer complected or caucasion it could indicate a number of problems especially with you being 18. You need to see a dentist to evaluate this and see whether is is just normal for you or something needing attention.

Blood vessels? : If it's under your tongue, see if it looks like blood vessels similar to varicose veins. To be sure, you'd better see a dentist for a definitive diagnosis. Unfortunately, we can't be expected to make an accurate diagnosis from what people describe to us over the internet.