Without detracting from the horrible insult...
Niger = Latin word meaning black.
Niger-brown would mean Black/brown or Dark Brown.
Admittedly a less "loaded" adjective might have been more wisely employed.
OK - for the last time:
The compnay who made the sofa, and its label, was a far eastern company. 'Niger' is a colour term that has been around for a long time, please see its use in botanical terminology for dark leafed / stemmed plants. The far eastern company took this term from wherever it is they get the european names for their colour palettes and put in on the label for the couch, but it got typoed by whatever person inputs the text in their company or, more likely, the outsourced print shop. Yes, in a western context you might reasonably expect a company to avoid 'niger' as a colour descriptive - but this was not done in a western context and therefore the same tutting and thumb biting cannot be employed.
It was an honest, and unfortunate, mistake. I'm sorry the woman and her family were upset by it but there was no malice anywhere along the line.
You try coming up with names for a colour range in Cantonese or Mandarin - I bet you 10 bucks in a list of twenty colours you'd get at least one
double-entendre or potentially insulting phoneme-string.