18% grey is NOT used to set white balance.
That’s news to me! You can set white balance with a black reference, any shade of grey reference, or a white reference. The software does not care, as long as the reference has equal values for red, green and blue. It works out the change required to make the point you select neutral and applies the change to the whole photo.
Shooting monochrome or colour, you can use almost anything that is at about the desired brightness to meter your exposure. Grass and sky would both work depending upon what you were trying to achieve with the scene before you. Concrete also works pretty well in many cases.