Pete, the situation arises when you want to keep one object coloured but make everything else black.
Since Viewports came along we adopted a system whereby we draw on Design Layers in colour (according to Class) and then when we present our drawings on a Sheet Layer we do one of three things:
1) leave as is, in colour
2) choose 'Black and white only' from document preferences, or
3) if we need some colour to remain, override all Class settings in the Viewport to black, apart from the object we want to remain in colour.
One real world example of option 3 is lease/conveyancing plans (or site plans) where, at least in the UK, the boundary line is required to be red. In a recent job I was producing lease plans and we had a coloured hatch indicating a floor finish. In this instance I had to go back to the hatch and edit it to be black, which we would rather have remained coloured on our Design Layers (for the very reason that we started using colour in the first place).
In the case above it wasn't such a problem because it didn't really matter that the hatch was now black (because it was just a visual aid), but I can see a different workaround is going to be needed on another job where we require the hatch to remain coloured (because it indicates some landscaping on a different drawing). In this case we're going to have to redraw the boundary in the Viewport Annotation and control the colour of it via the Attributes palette.