Give it time, good water quality, regular water changes to replenish calcium levels, and good lighting and the purple will grow.

You can also add a coraline "starter rock" that is nice and purple, then aim a powerhead at it, and it has been observed that this seems to help the coraline to spread faster.

Those products (Purple Up and other similar products) are widely considered to be unnecessary and of limited benefit, although some out there swear by them. Good rule of thumb is to try not to add anything you are not testing the levels of to determine how much is needed and whether it's needed at all. New tanks should only require regular water changes in order to replenish the nutrients they need. I believe you tank is still newish? Or am I misremembering?