Afaik yes, although I remember reading that (I think) Koko sort of asked something (I think it was "what color" or something like that). But at the same time I remember reading about similar criticism you mentioned, that Koko's signs were often quite random and the caretakers often tried to make fun of the situation that "she's just joking".

I should find that article ...

Edit: I don't know if it was exactly this artice but it was similar

https://bigthink.com/life/ape-sign-language/

Edit 2: or this

https://slate.com/technology/2014/08/koko-kanzi-and-ape-language-research-criticism-of-working-conditions-and-animal-care.html