discolouring at top of seedlings?

[wh1t3w1dow1]

any idea what could be causing this? seeds were put into FFOF april 19th, watering with well water. it's only a couple plants showing it (dinachem and critical+). it's an outdoor grow btw and plants have been outside from the start. coldest it's got down to was 45degrees.

 

[Lochinvar]

45 degree nights, most likely

 

[mr.brunch]

Yep, that. ^^
I wouldn't worry