daij, smk, gg5

"underling; henchman" doesn't seem right. smk gives 黒ねずみ as a synonym so I took the glosses from GG5's 黒ねずみ entry.
I don't think "colour" adds anything.

@@ -8,0 +9 @@
+<ke_inf>&sK;</ke_inf>
@@ -29 +30 @@
-<gloss>dark grey (colour, color)</gloss>
+<gloss>dark grey</gloss>
@@ -35,2 +36,2 @@
-<gloss>underling</gloss>
-<gloss>henchman</gloss>
+<gloss>deceitful employee</gloss>
+<gloss>unfaithful servant</gloss>

GG5

https://en.wikipedia.org/wiki/Brown_rat
The brown rat (Rattus norvegicus), also known as the common rat, street rat, sewer rat, wharf rat, Hanover rat, Norway rat and Norwegian rat, is a widespread species of common rat.

Even here:
https://www.pestworld.org/pest-guide/rodents/norway-rats/
The Norway rat commonly referred to as the street or sewer rat, is believed to be of Asian origin, arriving in the U.S. on ships from other countries in the 1700s.

The lit translation is "gutter rat", so the "street rat" and "sewer rat" given by Wikipedia seem quite appropriate.  Looking at yourei results, there's a lot of colloquial dialog, and these seem like the most useful glosses. "Norway Rat" looks potentially useful for a pest-control company translating their website (this is what comes up in English...).  Probably not so useful in literature.

@@ -21,0 +22,2 @@
+<gloss>street rat</gloss>
+<gloss>sewer rat</gloss>