From a fellow grower: Watering about every 3 days as recommended. Got good soil. Using LED lighting on an indoor grow. All of the plants seem to be doing really good nice and dark green and healthy looking up until this morning. One of them looks like it is getting some yellowish coloring in the leaves and maybe a couple brown spots? They are only about a week old.
Can I ask what good soil?
Might be too much nutrients in it for little seedlings.
Can you please tell us the brand and type? @Spiney_norman’s point is spot-on.
How much water each time?
The best method for seeing that a seedling has what it needs without risking root rot is to instead use a dome and to not water the soil at all. Use a clear Solo cup or a cut pop bottle and spray inside of the dome a couple of times each day. The plant can get all of the water it needs via the leaves from the humidity in the dome. Start watering a few ml a day when the plant outgrows the dome.
Roots need oxygen. All a seedling has is a single tap root. It’s difficult for the plant to extract oxygen from the soil if it is chronically wet, hence the recommended wet/dry cycles for cannabis. A cannabis plant will tolerate dry soil far better than it will withstand wet soil.