Generally its not a problem unless the yellow is progressing up the plant until the whole plant begins to pale. If this is happening to your plant its generally a lack of nitrogen and its very easy to under fertilize when your using organics. I reckon if your a complete noob to this whole thing you can't really go wrong with that as a starter setup because its almost impossible to get nitrogen toxicity or kill seedlings unless you feed directly out the bottle or something equally stupid when your using organics.