They look good! If you look at pictures of other young seedlings, you'll see they're not a dark, dark green but a lighter green. I think another few days and you can start to add nutes. If you don't have a ppm meter to make sure you're keeping the total proportion of nute to water at the right level, then start very light -- 1/4 of what the label recommends. Or even less. I found that my nute level for my adult plants ran around 1/3 of the label recommendation. So start at 1/4 for a couple of weeks, then 1/3, then maybe 1/2 and stay there for the duration of the grow. You might find you need some cal/mag added at some point (at least I did), but that should be about it. I used a 3-part nute system for hydro. Advanced Nutrients micro, grow and bloom.