Just being a near native speaker of German I would accept "Falls nicht".
Thanks. Hopefully someone from Duolingo will pick up on this and add this as a possible answer. I'd still be curious to know if one of them is 'more' correct than the other, and why?
I agree. @dvdtknsn: both solutions are equally fine.
I tried "Falls nicht, sind die passenden Farben dabei?" but it was apparently incorrect on the "Falls". Can anyone explain what the difference is between "Wenn nicht" and "Falls nicht"?
I'am speaking German. "Falls nicht" is as correct as "Wenn nicht" in this case! =)