As far as masturbation
is concened,it is a perfectly harmless and natural activity.
In fact lots of myths and misconceptions have been associated with it which are totally wrong.
The only problem with masturbation is that you tend to make it a routine affair by doing it daily,whether you get sexually aroused or not.
If you ar getting sexually aroused there is no harm doing it but if you do it just for the purpose of doing it,then it is an addiction.
Such types of addictions make you cut off socially,you are more engrossed in porn material,you don't do your studies and simultaneously depress yourself by developing unnecessary guilt about it.
In fact masturbation is good that you can't get any STD
with it and once you do it,you can engage freely in other works.
Do it occasionally and don't attach any guilt or depression
It doesn't lead to any future sex problem,as feared in general.
As far as PIMPLES
are concerned,it has nothing to do with your masturbation and is a normal phenomenon at your age.
You may consult a skin specialist for it's treatment.
As far as the brown colour of your skin of penis is concerned it is normal and just stop paying any attention to it.
Relax and enjoy your life.
I hope it helps.