Some of my sunflower seedlings are showing unhealthy looking
yellowish leaves, on the same plant with normal healthy looking green
leaves. Am I overwatering them, or this a sign of some mineral
deficiency, or disease, or what?

Ken Cohen

This might sound a bit obvious but, where are these yellow leaves? The
cotyledon leaves (the first to appear from the seed) often wither and
die allowing the stronger 'real' leaves to take over in the energy
production duties.

It depends. If it is the two seed leaves at the bottom then this is natural. If it is not them and it is the true plant leaves then in might be over watering, or it might be lack of food.