What def. could be cause the leaves to fade to a lighter color with the veins dark?

if it starts on new growth probably iron def in from the bottom nitrogen if the stems are going purple also its mag

I up my cal/mag at the start of flower or I’ll get that yellowing

Sounds like Mag deficiency as Creepy_Steve has pointed out. https://dutch-passion.com/img/cms/Blogs/mag-deficiency.jpg

You are seeing first signs of light stress or burn. They others missed your light is 480 watt at 19 inches. Little too much too soon. It's a big light and I use one similar. Have same issues if I get withing 24 inches.

Mg problems take 4-6 weeks to see them.. the problem started a month prior, though. if you take notes and thnk it's mg, look 30-40 days back for any adjustments you made to Mg that is fed. mg issue would also have some rusty spots or discolored spots. cant tell in pic it's too far away. can't see much from that far away... if the chlorosis is only occurring at top i'd ask if you boosted p or k lately? to much or either can cause new growth to be yellow at top - possibly thinner blades too. pH problems typically cause a smorgasborg of issues.. nearly imposssible to discern 1 problem. i doubt this is ph. using a reservoir or fresh water each time? if leaving water out for 24-48 horus, stop doing that. it's not helpful and only makes ppl feel better for no reason. actually a net negative to do in all but extreme and rare contexts.

That tigery striped leaves are 100 % magnesium issue, but what caused it ? Could be excess of nitrogen, then it blocks it. could be just simple lack, thats why calmag is a must suplement for few times during grow. Could be wrong ph for plant to absorb. Only you can diagnose, we can only guess.... Epsom salt if its not blocked by nitrogen would be simple solution. If i remember well, too moist pot for long can play such tricks in soil ...

Could be nitrogen...If leaves just lose color If u see any dots or cripsy top then is magnesium