I have a breeding group as well as a couple of single males in other tanks. They will generally start to color up around 3.5"-4", but this will depend on their tankmates and their relative size in the pecking order. Great fish, but when they get big, are more aggressive than the Mara Rocks spilonotus in my experience. My big boy in the all male tank is about 10" long - not the biggest fish, but holds his own.
EDIT: Thought you were referring to P. spilonotus (Tanzania) - also referred to as Insignis.