Looks to be from too much light artificial, or sunburned by not hardening them off properly. They should recover, plants otherwise look very healthy.

Mark

Agreed with above ^
Looks to be sunburn, and they'll be fine. I've never had an issue with mine that present with the same appearance.

Jay

In my experience, that coloration is caused by indoor lighting AND?OR nutrition imbalance (shortage of P). It should not be a problem. It is also part of growth pattern in some varieties.

Thanks for replies, I did grow them under lights so you guys are right, that didnt occur to me when i noticed the leaves.

Do the leaves turn back to their normal colour after time do you know.