I mean, assuming that Adams doesn't do his own coloring, I bet it's just that whoever colors the comics forgot the character was Black. As the article notes, it's literally the first Black character in the strip, and they had been absent for a bit, so it's likely an easy thing to forget.

Perhaps it was just an honest mistake? Comic strips aren't colored the same in different newspapers (imagine this discovery as a kid on vacation), I remember reading an article about how Franklin (the black character in Peanuts) appeared white in a number of distribution channels for a while.