I posed this question on Nintendo Life, and I think it is relevant on here as well. Was the Game Boy Color a sucessor to the Game Boy? Was it an upgrade (a la the DSi, New Nintendo 3DS, Project Scorpio, PS4 Neo) or even just a hardware revision (like the Game Boy Advance SP, DS Lite, PS2/PS3 Slim).
I say it was a sucessor, this Iwata Asks article says so as well.
http://iwataasks.nintendo.com/interviews/#/3ds/how-nintendo-3ds-made/1/0
"2. Game Boy Color™: A handheld game system released in October 1998 as the successor to the Game Boy system. It featured a color screen.
3. Game Boy Advance: A handheld game system released on March, 2001 as the successor to the Game Boy Color. "
Admittedly, I don't think any sales figures exist of the Game Boy Color, as it always seems to be lumped with the Game Boy.
Lifetime Sales Predictions
Switch: 151 million (was 73, then 96, then 113 million, then 125 million, then 144 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 57 million (was 60 million, then 67 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima