The Flood Fill series of nodes in Substance Designer is super useful set of nodes that take a binary black and white image as an input, and assigns an index to each discrete shape, so that you can do things like assign a random color to each shape.

Substance Painter does not have an equivalent filter.

I found myself wanting the functionality for stuff like, painting every window pane a slightly different colour, given I already had a black and white mask of the panes. So I slapped this together in about half an hour.

It can be downloaded for free here.