So "mereka" can also be used for objects?
I don't think it should. I would say "warnanya" or "warna keduanya".
Oh and I forgot to mention that I chose "...mereka berbeda" and it was correct, thanks.
You are not meant to use Mereka for objects but people will understand it.
Why is it wrong to use a counter?
Dua buah apel ini...
Dua apel apel ini" incorrect?
Lagipula tidak ada terkejut ketika yang biru berbicara dan yang hijau menyanyi .