Hardening plants leaves turning yellow

suffolk_blonde

Hi, I've seeded some cosmos plants, quite a large number as I have large number of seeds which was due to expire this year. I've seeded the plants in my loft conversion which is quite a warm room. I've started to harden the plants and take them outside but noticed with the Cosmos and morning glory the leaves have started to turn yellow - is this a natural stage or am I leaving the plants out for too long - I'm only leaving the plants out in the sunlight. 

Dovefromabove

Morning glory leaves turn yellow and then a creamy white in low temperatures and they stop growing. 

It’s too cold yet for them to be outside ... I wont be sowing  mine for a couple of weeks yet. 

I sowed my cosmos on Saturday. 

suffolk_blonde

Thanks for the advice, lucky only have tried it for two days while the weather has been good.