In DWCS6, what does a green DIV mean?

Here's a question I cannot seem to find an answer for, either in Helpfiles or online sources:

What does a green DIV mean?

In DWCS6, when you're working with Fluid Grid DIVs, sometimes the software will show one of your DIV boxes as green.... as having a transparent green background. Any text or image you place within that DIV takes on this green coloration. Do note that nowhere in the code view do I see any HTML or CSS coding specifying that it should be green, and it is certainly not a style that I have applied.

I think the green color is supposed to be DW telling me something, but I know not what.

Ideas?

Thanks, DAVE