Shiva Skunk Yellowing!

My plants look great since I took them out of the hydrohut. The growth has taken off and the buds are just now starting to form. I am in week two of flowering and last wednesday I pruned the bottom 3rd of all of my plants and watered with half strength flowering nutes.

Just yesterday I noticed some yellowing on the larger fan leaves on a few of my plants. Do you know what this could be? Here are a few pics:





You can't really see it because of the HPS, but there is some slight fading on some of the fan leaves in this picture.




Some of the yellowing:




Also, on a side note, all the stems are pretty much completely purple. Help?

yellowing on older leaves is ok , its the plant using up the nitrogen in them , in the later stages of flowering most if not all leaves apart from the buds go yellow. You can alter ferts if you want but that much yellowing doesnt seem like a danger , as long as you are feeding regularly u should be ok , be aware that the larger the plants are and the more growth they have the more nutes they will need.

Should the plant be yellowing this early into flowering? (week 2)