At 6 weeks (2nd week of flowering) buds are green but larger leaves are yellow. Is this serious?

Under fed. Feeding every second week is nowhere near enough. Nothing to do with pH, humidity or soil moisture levels.

yellowing leaves from bottom to top is usually a sign of nitrogen deficiencies, could also the result of a nutrient lockout or ph, it all depend how you've fed them so far ...

leaves don't function well if they die, so kinda important to provide energy to the plant to power growth/ripening. if it paled starting at bottom and moved up, that's N deficiency (also tip and in progression on leaf itself.) Looks like you need to feed more, and probably across the board unless you recently reduced N proporitons of your mix. The mars light you are using is 100watts. 2.5-3sq ft is probably it's max, so it should have done better.. you likely had the light too far away. go for about 18" next time, but observe and react to resulting internode leghth (the anatomy of stem between 2 nodes). If too long, get it closer, if too short, pull it back an inch. So that the light works well late in flower, vege at about 66% power over 18hours (vs 100% during 12h/12h flower). This way you only need to bump power to 100% and keep same hanging distance from plants when you switch to 12/12 cycle. If autoflowers, may want 18-24" and 100% power entire time you can cover more than 2.5sq ft with 18hour operating time and a bit more distance from plant. again, use the resulting internode lengths to guide your adjustments based on growth pattern. that gnat problem could be control by preemptively using a product with Bti in it.. bacillus thurgin..something or other... there' won't be anything similarly named as far as the bacteria used.. Bti is the short version. Mosquito Bits by summit brand is the one you'll find in north america. bits are a top dress and dunks float in a reservoir. apply every 2-3 weeks. Substrates are not often stored well by the sellers.. they stack them outside at some point and they get larva and all sorts of other petential infestations. use bti from day one and you won't see any.. maybe 1 occasionaly but they will never be enough to damage roots or escalate. adult gnats don't cause problems besides being annoying and reproducing, the larvea damage roots. yellow sticky traps are good to leave up even if you don't have a problem.. you'll find instead of 100s there'll be 1-5 after an entire grow cycle using Bti. Simply top dress some bits and as you irrigate, it awakens the bacteria from dormancy and it seaps into soil whereit will prevent larva from surviving in a highly effective way. using this product mid-problem would take 2-3 weeks to stamp it out. Much better to use it preemptively. this bacteria or a subspecies of it, has been used on the world's food crops for many decades.. it is 100% safe.