coralline algae is what turns the rock pink/purple. Your rock is going to get ugly before it turns pretty. If its a new tank I would wait till you go thru the ugly stage.(diatoms, hair algae etc..) After that get someone who had coralline algae to scrape some off of their glass or rock and use it to seed your tank. It takes time and i wouldn't worry about it till the tank ages at least 3 if not 6-8 months after the cycle is complete.