First pair of leaves losing their color and growin...

Didn't see much curling or yellowing in the pics. If by first set of leaves you mean cotyledon's, then it isn't a problem at all. The cotyledons are there to provide nutrition for the seedling through its first stages of life. Many growers won't start feeding their plants until they start to see those turn yellow. Happy growing! 👊

As I see you are using a GC4 you shouldn´t have any heat stress, in this seedling stage i think you only use the grow-switch which produces about 32W of light. In my opinion it could be a low air humidity which should be over 60% in this early stage. But what I see you still use a humidifier. Maybe it could be wind burn if the vent blew her too much. If it really should be a heat problem, lower temps down to 25°C max.

The most probably cause is the light been too close, wich will cause heat stress, do increase the light distance. Also wrong ph can have that effect.

I found the best thing with seedlings is to cover them with a plastic dome (you can use a 2l coke bottle cut in half) and spray daily inside the dome this will keep the leaves moist and the light in. I think if possible raise your light to 35m40cm or dim it off or has that option, they don't need much light when so young

Heat Stress Heat stress causes cannabis leaves to start cupping (turn into "canoes"). Another symptoms is leaves pointing up Problem: Your cannabis plant can only withstand a certain amount of heat and light. After a certain point, your cannabis will start exhibiting signs of stress on the leaves near the sources of light and/or heat. Your leaves will get yellow or brown brown spotting and may appear generally burnt in places when there’s too much light. It’s also common for leaves to curl up or down, fold inward like conoes or tacos, and for the serrated edges of leaves to start flipping up. Solution: find a way to lower the temperature and/or increase the circulation in the grow room or grow area if heat is the problem. Having a small fan blowing over the tops of your plants will help prevent hot spots from forming directly under your grow lights. How far away should you keep your grow lights from your plants? You may consider removing grow lights further away from the tops of the plants if heat is a problem. When growing cannabis, it’s best to try to keep things at a comfortable room temperature at all times for optimal growth. If it’s too hot for you, it’s probably too hot for your plants. Keep roots cool! If you can keep your roots cool, it will help your plant deal with heat affecting the top of the plant. If there’s some way to protect the roots from heat, do it! Organic Liquid Sea Kelp Extract can help cannabis plants recover from heat stress, extreme environmental conditions, and may even help plants be protected for future heat waves When cannabis plants are recovering from heat shock, some growers recommend using seaweed kelp extract (often available as a convenient liquid fertilizer) to help plants recover from the stress and possible even protect plants from heat stress in the future. Many indoor setups will require that you vent out hot air using a fan and/or an exhaust system. By creating good suction with an efficient exhaust system and adding a carbon scrubber, you can also pretty much scrub all smells from the grow room. An oscillating fan will circulate air in the room as well as provide a gentle breeze for your plants, and a small one will cost less than $20.

Light is too close, the seedling can’t stand the evapotranspiration.

The Answer is : Heat stress, Root Problems or Extreme Humidity, so cheak the temp and Humidty in the room if they are too high ,get ur self a Fan , if they are both ok then u maybe watering too much , the seedlings are very sensitive at this Stage

hmmmm i was looking at the pics, and i see atm 3 possibilities....... wich soil are you growing in =? is is a mj fitting mix=?. second question........did you move the lights nearer=?....... third question... do you see little bugs under the leaves=?... its so hard to tell because its the first week and i have not many informations. If you have a nice mj fittig soilmix we can exclude a lockout= or did yu feed them=?.. leaves looking spreckled thats either of the lights to near ( lightburn) or having mites...... for solving your problem i need more closeups to see the lef structure. My 2 favourites are wrong soil or plants to near on your high quality led. Because greenceptions got a big lightoutput , and those little seedlings just need alittle light in the first 2-3 weeks... the longer i look i go with a lightburn.. so pls add some closeup pics and we exclude some more possible problems