Brownish yellow Rocks

[asylumdown]

rocks don't just turn purple. The purple is a specific kind of algae, and you should see it growing from small little spots that gradually expand to become large colonies of purple coralline algae.

That's not to say that marco rocks don't change colour as they age in a reef tank. They should go from bright white, to potentially limey green to yellow (that's a phase of early colonizing micro-algaes), and then eventually turn a base grey colour that looks more like normal live rock as microbial process on and within the rocks stain them. However, the colours associated with live rock are 9 times out of 10 due to various kinds of coralline algaes that encrust all over them, and there's many different species. There's the purple kinds, but there's also brown, orange, reddish and pinkish kinds.

Without pics it's hard to tell what's causing the colours on your rocks, but if the rock that you go to 'seed' your marco rock didn't have any of the purple species on it, you're not going to get any purple on your marco rocks.

FWIW, I have a tank that's 50/50 marco rock and Walt Smith rock. I've noticed that the orange species of coralline algae are way faster at colonizing than the purple, and the purple species don't seem to love super bright light. But in any case, if the colour change is a result of coralline algaes, you should be able to see the discrete edges of the individual algae colonies. coralline algaes look a lot like patches of lichen that you'd see on rocks on land, only way denser.