Little spots and slight yellowing. What is the cause?

I see leaves with consistent color end to end... a mostly healthy plant. i would not react to anything you see at the moment. The first 1-2-5 fingered leaves often have blemishes you'll never see again -- assuming properly fed of course. Easy to verify a good light distance -- how are the resulting internodes? internodes are the length of stem between 2 growth nodes. If they are sturdy and strong but not too tight, you are fine with your light. If lanky, stretch you need to increase light intensity to the area of coverage one way or another. distance or power can be used to dial it in -- distance is better used to focus light on area of coverage, imo. some contexts may deviate, but this helps you stay on target better with intended DLI - daily light integral. worth a search get a leaf symptom chart and start lookingn at it each time you see something and compare... even if asking questions here. it takes time, but doing so willmake you familiar with all the symptoms and understand that many symptoms have multiple potential root causes. Always be certain before reacting. Any context where that isn't true is probably a dumpster fire anyway, lol. slow the problem progresses, the smaller the adjustment needed. You haven't fed yet, so it's probably just the soil starting to lose its nutrient charge. take note of when this started occurring. if you buy same products, start supplementing a little fertilization in a preemptive fashion and avoid it... You are about to fertilize, so you'll get some good feedback soon. I'd wager any symptoms dissipate. And, if you ramp up appropriately and give a proper ratio of nutes, the plant will look fine... 1 step at a time...

If your at full power 2x100w and 20in you probably want to back it off or dim the lights. I wouldn't do anything but continue to monitor it, your next feed is a nutrient feed anyway, looks like substrate's drying out getting close to feeding time anyway. If you start messing with your Nute schedule before you even start using it you can create more headaches for yourself .Feed your solution next feeding if it goes away you know to start a little earlier next time, if it continues to progress then make an adjustment. It's easy to panic at the first signs of something and over correct, you can wind up just fighting yourself. Plant looks fine as of now, take it easy and monitor.

Looks like a magnesium deficiency starting. You can put a tiny flash light under leaves, and see spotting through a leaf better. Also If your lights are both on 100 percent it is too intense at 20 inches for vegging stage as well. I would back off a good 16 inches if you can't dim them, and or need the warmth from them being on.