red/green (or red/blue) with more than 30 million sales, if you sum with yellow, we got more than 45 million sold
Wow, that's really impressive.
But with B/W 2, maybe B/W series can surpass this outstanding sales number.
B/W got 14.71 mi sales, around half of that number.
And I don't discard the possibility of a Gray version
you cannot consider Black, White, Black 2, White 2 and Gray as same game and compare with PKM Red/Blue. By the way, B/W has worse sold less than Gold / Silver, less than Diamond / Pearl, less than Ruby / Sapphire, less than, etc.
I cant call it a failure because it was cheap produced, and have good sells comparing with games in general, but comparing with pokemon series, it had a weak sale performance (as BW1 it is dead already, we can take this numbers as final numbers).