They look fine to me - the first true leaves being a bit lighter than the cotyledons is normal.
The cotyledons contain the nutrients needed to get the plant at least to the first true leaves, and some say to the 2nd. I wouldn't be giving them anything just yet, but that's just me.
Yellow is usually a sign of overwatering, not under-fertilizing. But it's moot at this stage - those seedlings are so new, you probably need to just step back and let them grow a little.