Calcuim has never colored my stools before. As Inape pointed out, bile can cause yellow or green stools, and you have a problem with bile either with or without a functioning gall bladder (just because yours is still in doesn't mean it's functioning!).
But have your stools been green before? Do they burn? If yes to either, this may indicate the source of your D. If not, then look at your diet. Loose stools are easily colored by what we eat. Have you been eating spinach or other leafy greens? Peas? Drinking Kool-Aid or colas or ice pops with green dye in them? Eating pistachios (no joke, pistachio ice cream will turn my stools green)?
If you can't find a food reason for this sudden color-change in your stools, you might try stopping the calcium to see if they go back to a normal color. That they color your stools doesn't mean that they're harmful to you, but most people do like to pinpoint what's making them abnormal. Pepto Bismol will color people's stools black; it's not harmful, but it's one of those things you want to know about and understand, even as you can safely ignore it.