My Seedling turned yellow. What's going on?

You need to be adding nutrients at this stage with a light mix mate. 200ppm not enough. Usually id say pigment but I see what you talking about. You need to start adding a bit of grow Ph keep it between 6.5-6.8. If your tap is at 200, you've hard water. So, you won't need a calmag too early. But will be good to add some Epsom salt for mag/sulphur, its the best for soil. And any decent nutes should of calcium in it Get a soil nutrient. Many nutrients will give ph of 6ish. So have a look at what you getting. You wanna keep your feed as close to your tap, feed, water, water feed. Etc (water feeds can have no salt ec nutrients "core NPK grow etc") Add 350-400ppm for the 1st 14 days or so and start upping it or give feed more frequently. You'll find your stride I'd advise with soil getting an enzyme, something with benifical bacteria as that's the point of growing in a soil.