If they are green under the skin, you're supposed to throw them away because it means they are producing this stuff called solanine, which can make you ill. Supposedly though, you have to eat a large amount of potatoes/solanine to get sick/poisoned.
If you Google "green potatoes", you can find some pretty reputable sites with info (universities, etc.).
I've had them before where the skin seemed tinged, and I just peeled it off and was fine. I'm not sure if eating the skin would've made me sick though.
When you say they don't look greenish in the store, but they do at home...are you using them right away? Potatoes can go bad quickly if they're not kept somewhere cool, dark and particularly, dry.
<<<hasn't yet met a potato that she doesn't like
As for the garlic, I've not seen it greenish ever, but um, yeah. Blech.