Donny X Grapfruit

The colors will come out if the genetics allow for it and if the plant was well cared for. Some plants don't turn colors. Purples are induced and encouraged by dropping nighttime temperatures quite a bit.

I read somewhere that if there is a difference of temperature (not more than 8 degrees Celsius) from night time to day time it increases the chances of colours showing. Flushing towards the end also should bring them out more.

In my experience, cold dark cycles seems to bring out more color.