I would buy a very small amount of true live rock.. probably about 10% of the total amount of rock in your tank would be a good ratio of live rock.
The purple up you are referring to is junk. Don't waste your time and money with it. The coralline will come naturally and then you will curse at it every week when you have to scrape copious amounts off the front of the glass.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor