Cotyledons turning purple around edges

[scrufy]

So I have a flat of 72 germed and just starting to get true leaves. I noticed the last couple days that a lot of the plants cotelydons are turning dark purple around the edges.
I'm running T5 HOs about 8" above them in rockwool plugs and bottom watered with no nutes or fert.
Are my lights too close maybe?
Any ideas?
I've never had this happen before.

Thanks

 

[Portuge]

Sounds like a suntan....

 

[scrufy]

Yep thanks for confirming.
I figured it might be just the lights are too close.
I just never saw this in my seedlings before.
This is my first year using the T5 HO setup so I have to adjust.
I always just used CFLs before.