When they're first born, their poop will be green. Since you got your chicks from the feedstore, they are probably a couple days old already and their poop is most likely going to be brown with a little bit of white. The white is their pee. The "foamy" poop that you are seeing is called cecal poop and it is normal, usually it is expelled every 8 poops or so.
You may also notice some poop stuck on their vent (butt). Wet a paper towel with warm water and wipe the poop off. Be very gently as you don't want to tear their delicate skin. After they are a week old you shouldn't have to worry about pasty butt anymore.
Definitaly raise the heat lamp if they appear to be too hot.
Good luck with them, they're adorable!