Why are my tomato plant stems turning red?

crayola

I'm growing cherry tomato plants from seed, and having got them to the requisite 10-15 inches, am now putting them outside on the window sill before transplanting them.

However I was surprised to see that after the first night I left them out, their stems had turned a purply-red colour. Anyone know why this might be? I'm pretty sure they are not supposed to be red!

thanks all

annie123

Too cold for them.

You need to harden them off properly.
Take them outside in the day and in at night for 2 weeks, then when you leave them outside at night for the first few nights, provide some cover for them, either plastic bottle cloches or fleece over them 'till they acclimatise.

crayola

Fair enough they did already look marginally less red this morning after a night back indoors, so that would make sense. Should they make a full recovery now!?