That's a nute problem. The middle photo looks like magnesium def, with that golden yellow spotting. I've never seen it quite so advanced and if that's it, it could be why the yellow spots are turning brown. You're probably seeing alot of golden yellow spotting at first that turns brown I bet. Avoid flushing. The best way to avoid nute problems is to do what's necessary at the beginning and avoid changes. The best way to avoid nute problems, in my opinion, for a beginner is to use the powdered ferts like Foxfarm fruit and flower, tree frog, Dr. Earth. These ferts have the bacteria and fungi necessary to break the large molecules down to a level that the plants can absorb. One half to one cup of fert for every two gallons of soil is plenty, one cup is actually too much but it won't hurt the plant because it takes time for the bacteria to break the stuff down, so think of it as a natural clock, feeding at the right time. Some people like soils like Miracle Grow but it's kind of hot to use and draws a few complaints. But some people like using it.
To attempt to solve this problem soon (and if I'm right about it being mg def, the a spoon full of epsom salt in a gallon of water should do it. Just water until they have enough, you don't need to give the entire gallon.
Another option is to try a fert that has pretty much everything in it, like Supernatural Terra or Dynagrow. It won't take much, just follow the directions on the container. The thing about soil grows is that the manufacturers don't necessarily say how often to feed. You could use a Foxfarm liquid fert or Botanicare and follow the feeding schedule, but I don't recommed this for beginners. Putting powder fert in your soil and letting nature do its thing is best. Just add water when dry.