Why does this shampoo leaves build up and oily

I myself don't get any build up. Although I probably use a little more than recommended. I add some, then more until I get suds. If I don't, then I get oily hair like I didn't even wash. A helpful too is using a hair scalp massager/exfoliate with it.

Is this ok for color treated hair ?

Yes the Nettle is safe for use on color treated hair.

does this shampoo have a fragrance and if so what? Thank you

Yes, the scent is from the Nettle plant. Warm Regards, Be Pretty

How much shampoo do you use per wash?

Hello there, thank you for your inquiry. Please be advised that it is all dependent on how much hair you have. I would also advise double washing so that the first wash washing off the dirt and the second wash reacts as a barrier. The second cleanse will foam up more. I hope this information helps.