What is a red spider with a green butt?

A "Black Widow" has a red hourglass shape on the underside of its abdomen, but it isn't impossible to spot a spider with the infamous red mark on its upper abdomen, that would make it a "Red-Back Widow," which is found only in Australia (unless something has gone horribly wrong!)

If you don't live in Australia, it is most likely a "Northern Black Widow" which has several variations in the location and size of the red mark, usually small spots or the hourglass shape.

If neither of these is the spider you're asking about, consider the hairy jumping spider, "Whitmani" The female is black or dark gray with a bright red abdomen