I can see nothing wrong with grey, in itself. We have painted our bathroom grey, but it is when you see so many houses where everything is grey: floors, walls, curtains, furniture, cushions - and every single room in the house is the same monotone colour, with no other colour to relieve it.
At present it is not just houses, hotel rooms, holiday rentals are all decorated and furnished throughout in the same monotone.
It was just as bad when we had the 'cream dream', and before that when everything was white.
As a character in a G&S opea says: Yes, we all love toffee, and toffee in moderation is a capital thing. But to live on toffee - toffee for breakfast, toffee for dinner, toffee for tea - to have it supposed that you care for nothing but toffee, and that you would consider yourself insulted if anything but toffee were offered to you - how would you like that?
For toffee read 'grey'