It will redden after first frost. Since you may not have a first frost, it will stay greenish-reddish - and more bitter than those that get frosted. It is a matter of taste, I like it equally, frost-sweet or bitter. In its bitter state it is medicinally a lot more powerful (helps your liver more). Radicchio is really optimal for zone 8 or 9, minimal frosts, but frosts. Michigan radicchio, around Xmas just before it keels over, it may be small because of the short season and cold temps, but it is the sweetest.

There are self-heading varieties that I think have inferior taste, and I suspect that the tasteless heads you find in stores are of that type, grown somewhere near the Rio Grande or in Salinas.

Yes, less sun causes greener leaves- chlorophyll (the green pigment) is more effective at photosynthesis than the red pigments, so with less sun the plants will make more green pigment to use the available light better. I would test it and see how you like the flavor. If it is too bitter maybe blanching it by wrapping it in brown paper might help, though I don't really know. It looks very happy and healthy otherwise!

hi i live in Grafton n s w in subtropical climate and i get no frost my radicchio is green like lettuce with large leaves like silver beet but still taste great so why worrier just enjoy