Yellowing leaves? HELP!

I'm helping my friends with their little operation and noticed that the leaves on some plants were yellowing/becoming a lighter, yellower shade of green. I'm posting some pics hoping that somebody will know what is wrong. They currently have 2 1000 watt HPS bulbs and are working on buying venting and more fans. For now, I'd say the room stays at a constant temperature of 80 or 85 degrees Fahrenheit. The humidity feels high in the room and I'm trying to convince him to buy a dehumidifier. Let me know what you guys think please! Need some professional help!You can see in the second picture how one of the plants is a natural dark green that I've seen before and the other ones look light green/yellow. Thanks for taking the time.

looks like lack of N... to me but i'm not a pro! ... what stage are they at? ... looks like late veg/early flower ... have you been feeding them? if so what how much and how often... what is your light shedule currently?

could do with a closer look at those affected leaves to get a better idea

if you have switched or about to switch 12/12 then "your friend" is going to have to get air shifting in and out of the grow room/space ... if you move enough air then it should sort both the temp and humity problem... (unless local climate very high humidity you shouldn't need a dehumidifier) ... there is kit you can buy that control the can type fans to keep your space at the temp you require... or you can just buy the fans and ducting and adjust them manually to get results cheaper... you need to do this or you will have nasty mould problems when they bud and all "your friends" hard work will be wasted

i'll say it before someones else says it with attitude.... there is stacks of questions that have already been answered that can be searched for on this site ... its the best site around imo ...but saying that give us a little more info and i'll give you my guess

what size pot, nutes, water schedule. are they in flower......?

The first pic looks like mag def. Unlike a magnesium deficiency, nitrogen def will start from the tips and work its way back to the leaf node. Nitrogen and Magnesium get confused. The best way to tell them apart is, nitrogen deficiency starts around the tips and works its way to the back of the leaves, where a magnesium deficiency will cover the entire outer part of the leave and make the entire leaves yellow leaving the veins to stay green.

Ok, I'll tell my friend to start adding more CalMag and update you guys. Thanks! And I'm trying to tell him to lower temp as it is usually around 80-90 degrees F. I think ideal temperatures are around 70 or 75 degrees for flowering.