On "gamps" – the word

The thing with being a non-native speaker is that sometimes you don’t even reflect on strange words. "Gamp" is what it is called, even if I would have tried with "colour sampler" first
It seems to me that "sampler" (or just "sample") is any sample, but a "gamp" always is about colour. Am I wrong on that? Or can a "structure sampler" (like, for instance the draft here) also be a gamp?

Somewhere I’ve heard the explanation that it comes from Sarah Gamp, the Dickens character. Now I decided to try to look it up.
Dicitonaries of course gave nothing – there is no English word spelled "gamp", just so you know...
Next try: google.
I got lots of hits. ALL of them (ok, just the first 20 or so) told me that is is British slang for umbrella, named after Sarah Gamp (see above). Unless, of course, it means Good Automated Manufacturing Practice.

On Wikipedia, I found that Sarah (or Sairey) Gamp "habitually carries with her a battered black umbrella".

So, dear readers (especially the native speakers among you) – WHY does a BLACK (and battered) umbrella lend the name to a COLOUR, er, "gamp"?