Why are my cauliflowers going purple?

This is often a symptom of something lacking in the soil, probably one kind of mineral or another. It shouldn't really be a problem especially if the plants are growing well in other respects.
The other possibility is that there is some damage to the root system, possibly from cabbage root fly, but if the plant is growing well it is unlikely to be this.
Are you in a new garden? Because the other very slight possibility could be clubroot, but this is unlikely on clean (new) soil or a soil that is alkaline rather than acid.
I would be optimistic and go for the first suggestion!

Purpley plant leaves generally indicates a phosphorus deficiency which could be from a soil that is too cold, too wet, too dry or maybe, possibly a general phosphorus deficiency in the soil.

and of course they may have been mislabelled and you have calebrese or some other brassica.