The Black & Decker Power Scrubber's use is limited to stuff that is
hardly dirty. When not in use, the brush is hung upside down above the
sink.

The hard bristle attachment to the same Power Scrubber shows the same
thin crusty coating of orange crud. Like another poster said, maybe it's
the plastic. I have been wanting to mod a different scrubber to the
device, this will be a good excuse. To test whether or not the unrelated
attachment turns the same color. I hadn't thought of that possibility.

I tried soaking it in vinegar. Then soaked it in bleach. Then in a harsh
cleaner/degreaser called "Purple Power". For many hours each.

Plastic is amazing stuff, we make some great plastic here in the US.
There are very many types of plastic. As complex as it is, it probably
can be weird stuff too.