stuart noble wrote:

I have seedlings that went white a day after putting them outside in a
cold frame.

Sunscald; put them out with some partial shade for a few days as they get
acclimated to _real_ sunlight.
At one point I got a light meter to determine what my artificial lights
were doing. It was shocking to see the difference between outdoor
sunlight, sunlight in a double-wall polycarbonate greenhouse, and
fluorescent grow lights!

--
Gary Woods AKA K2AHC- PGP key on request, or at home.earthlink.net/~garygarlic
Zone 5/4 in upstate New York, 1420' elevation. NY WO G