Aren't they naturally coloured?

There are 2 Croton (Codiaeum) species native to here, Codiaeum membranaceum and Codiaeum variegatum var. moluccanum. (However, Kew seems to be putting all varieties into synonymy.) But these tend to have all green leaves. The cultivated form of Codiaeum variegatum is said to come from Ambon and has very unstable colours. Breeding and cloning of abberant colour patterns has resulted in the production of hundreds of different looking plants. So they are natural colours produced by what are defective genes that are passed on to successive generations. And humans are making sure that natural selection doesn't diminish the survivability of those defective genes.