purple shaded leaves

I am new to the site, but I am not new to forums. I'm sure you veterans are tired of hearing the same questions again and again. I have searched the entire site and a few other ones, but haven't found any answers to my question yet. I have 10 White Widow seedlings, 5 were germinated in standard potting soil and 5 others were germinated in potting soil with guano/manure for fertilizer. They are all growing very nicely so far and are about 3inches with their second set of leaves. The non-fertilized ones are a lush green, but a few of the fertilized ones are green with a very light shade of purple around the center near the tip of the leaf. The stems are not turning color and neither are the veins under the leaves, but I'm a little worried still at the difference between them. All growth looks healthy. Anyone else run into this as well???

Its probably the strain! Or its too cold in your room, which usually triggers some purple in the leaves. Im not quite sure though, its nothing to be worried about. just keep the feeding nice!

Wow your reply is great thanks! My grow area is always around 75-80F so judging by that things should be okay. Thanks again Sensiskunk! :)

hay saw.. have you look at the phosphorus leaves? plant's with low leaves may develop a purple coloration or so is says in the book's hope it help's ps: its saying it mite slow plant development as well stunting the plant growth..