It's probably transplant shock, the sudden change being brought outside. Purplish leaves indicate cold weather. Keep them well watered & they should recover in a few days.

In the future, when you bring the plants outside, let them sit out for a couple days so they get used to the outdoor environment. Then plant them in the ground. If you still have some seeds left, plant a few in the bed. They should sprout up quickly with no problems at all.

this sounds as if they have got frosted. Did you harden them off or were they hardened off when you bought them?

All I can suggest is if you know there is a frost forecast cover them with some fleece for the next couple of weeks.
They may recover and become strong plants .

welcome to GoY too.