hi ,
we had a tread about this a few weeks ago on our french forum.
the red color of CPs come from certain molecules , called anthocyanines
anthocyanines are produced by plants in response of an "agression" by the sun , specially from the UVb light.
that's why a plant become red when it's put in the sun (or under strong light)
but : VFT also produce anthocyanines when the weather become colder (also S.purpurea do that : at the end of the summer , S.purpureas are much more red than in spring)
how does that works ?
anthocyanines are produced by the plant , in response of the cold, by using the glucides that it have accumulated during the day...
wich glucides ? the one produced by photosynthetic activity during daytime (the plant "feed" from these glucides , but also from other nutriments ... the insects)
why ? because if the plant dont absord the glucides that she produce , she "kill" herself by overfilling her chloroplasts with non-used glucides....
that's why you need a combination of UVb light , and cold temperatures (at night for example)
in summer, the metabolic activity of the plant is "fast" enough for all the glucides to be absorbed , when the weather become colder , the metabolic activity is slower : the plant dont have "time" to absorb the glucides produced the day before...
you can also try to give more UVb light to your plants by using Wood bulbs (sometimes called "black light" , the same used in dancingclubs : it make the white colored things glowing in the dark)
was it clear ? ok , I'm french , it's a bit hard to explain things in english, sorry
patrice c