What is best to do when plant is turning the color...

i don't think it can be a deficiency that early. Coco coir? if not buffered or initial fertilization, maybe a dose and see what happens, otherwise i'd guess there's too much of something rather than a lack of something that early on in the life cycle. I nearly killed some clones with cheap coco that wasn't washed and buffered properly. Took about 40 days for new vegetative growth to start. I think your week 3 comment is correct. Some stretch on the one for sure. Also, cold can inhibit how nutrients are used within the plants - as can pH and other things. So, it'd be very difficult to deduce which it exacly is if it comes down to that. Tons of variables to rule out. Many with your recent behaviour. Since it's stil unclear, i'd wait longer.. irrigate with pH only, try to get them warmer, if cold still. Light does of ferts of you think the coco is completely devoid of nutrients due to what you've done so far, and if so, after that hydrate only for a while until roots colonize a greater portion of the substrate.

Try putting a clear solo cup with water misted on the inside over them to help raise humidity! They will love it!

To fix allow medium to dry out well before next watering. U can put the pot on a seedling mat to help evap some water off. Or something of similar principle to a heat mat. Something to warm the medium slightly to evap excess moisture.

Roots are sitting to moist . Oversaturated soil doesnt allow enough oxygen to the roots. Resulting in the yellowing u have. Haw is drainage in the pot? Well the seedling is young dont fill to run off(completely saturate soil.) Water around the edge of ur pot. To stimulate the young roots, forcing them to expand and search for water.