Yes, they probably do need nitrogen, and if you used "regular"
5-10-5 MiracleGro for the herbs, you're going to get more flowers, not more green. You need a higher first number.
Yellowing, I think, is a symptom of iron leaching out from too much watering. Or maybe even from too dry of a climate, although mine just look droopy when we get too little water. Mine are not doing great this year, because of the cool weather (and my late start!). In fact, only 2 of mine "took" but those that did, look bright green, but small. I only use fish emulsion (organic) and that makes everyone happy - tomatoes, flowers, and herbs.