3 week old Skittles plant

There fairly small still they seem to look OK just keep an eye on them.if yellow gets worse may need check pH 👌

You're not feeding them at the moment, is that correct? I don't see any nute information in your diary so I would assume that to be true - and if so, they're at about the age where starting nutes would be a good thing. Start them slowly, 1/4-1/2 strength to make sure they're going to tolerate the nutes. Good luck!

light excess of nitrogen, which plays with magnesium. either you feed too much, or medium was too rich.

I don't think Is a problem, anyway check under the leaves for afids, and use neem oil if necessary