Leaves dying....

My plants are just entering their third week of flowering. Uk Cheese and White Rhino are the two specimen s
growing. Three days ago I noticed on the base of the plants that the a single leaf on two of the White Rhino plants had turned a yellow colour, and the remainder of the leaves are now a dark green colour, as appose to a vibrant green colour. Is this something to worry about ? Also at what heights should the lights be at, currently I have three 600watt Sodium lights for eighteen plants, at a height of 1.2m ? I am attaching a couple of pics of the leaves, and would really appreciate some help. Thank you.

Nothing to worry about as long as your new growth looks good.

They look like they are the first set of leaves after transplant.

Some white marks on your leaves, Have a look for Thrips.

Also looks like the beginning of a mag deficiency. Looks at some pics/descriptions of mag deficiencies, and see how closely the match up. Give it some CalMag if that seems to be the culprit.

To all who replied, thank you..

i was gettin alot of that on my lower parts of the plants my leaves wer drying up and dying burtthis was just because they wer gettin no light

ive just harvested my cheese have a look and tell me what u think

https://www.rollitup.org/grow-journals/277194-cheese-journal-2nd-grow-42.html
harvest pictures r on page 40