the yellowing is from lack of nitrogen usually.. at about a month old, it's time to start feeding them imo.. usually there is enough nutes in the soil to last a good few weeks so that you don't need to start feeding them right out of the box...but seeing that they're starting to yellow, they're telling you that they're hungry and are ready for nutes.. some plants will need nutes sooner and some can go longer without.. this is my first run with a new strain, c99, and they went yellow on my in about two weeks from seeds, which was crazy quick, but i started feeding them, and they look much better..
when you start to feed them, only give them about 1/4 of what it says on the side of what ever bottle you're using.. nute co's are in business to sell more nutes, so they usually recommend a hotter dose than what's needed...
every week bump them up by another 1/4, so after a month you're at the full dose on the side of the bottle, but if they start to get nute burnt before you reach full dose, simply back off of the nutes some and keep it there..