Seedling!

My seedling is growing by night, the cotyledon leaf is grown out, and you can kinda see the first 2 little fan leaves growin. but the problem is that it has a purple color right under these leaves.. is that normal?

I wouldn't worry about it too much at that stage. Yes, purple discoloration can be sign of a trace mineral deficiency ( can't remember which one off the top of my head ) but seedlings depend on nutrients from cotyledons to start out life so deficiencies can't really happen while cots are still attached.

That can also be caused low temps.