I'm 25 and in the last 9 mo. I've gained an extra 2.5in in penis length and when erect it turns a purple color. What can cause this

At 25 years old, nothing should be growing on you, let alone your penis, unless you have taken some of those penis enhancement drugs. If your penis turns purple when erect, it only means a lot of blood is rushing that way to accommodate. Now all you need is the one eyed monster, your have the purple penis to go with it.
Tick

I very much doubt you gained 2.5 inches in penis length at 25. The only way you could do this is if you bought an extension. Having said that, if the extension is purple, that's the cause.

Yes, I would be somewhat worried if I really had a 2.5 in growth > and also worried that people at age 25 are really taking time to measure it when hard.

*** I can see it now, wait a minute dear, lets stop for a minute and grab that ruler?? ( LOL)

But may be worth having your doctor review it

Purple head is too much blood rushing to your penis. Have you taken anything that increases your metabolism, i.e. weight lose pills, vitamins, too much (way too much) caffeine, is your partner turning you on more than normal (not making a joke - I get over excited sometimes to the point where it hurts from being too hard). Also, have you really grown 2.5 inches, something could be wrong, have your doctor look at it. If after that there's nothing wrong then be proud of it :)