yellow leafs

hi everyone, quick question this is my first grow, I've been veging for 5 weeks everything looked fine but first few days into flowering and some of the plants are starting to go a yellow green colour I'm
2 weeks into flowering they are having 2ml of nutrients to 2 leters of water in 30 leter pots and the colour still hasn't changed, they still seem to be growing ok though, there under 2 600 watt lights 6inch out and 4 inch in
about 18 - 20 inches away from the bulbs the temperatures are good, i don't know what the ph is as i haven't got a ph tester. here are some pics any help would be great, many thanksAttachment 286757 Attachment 286758 Attachment 286759 Attachment 286760