What color should my walls be if I have a gray couch?

Depends on shade of grey, how much light the room gets, etc. Can you post a picture? Short answer...your favorite color.

I would choose a rug, curtains or pillows with a pattern that has the colour of your sofa, and choose a complementary colour from it. You could also look at fabric remnants, a favourite blouse, even a magazine cover that has the grey in it. Designers are hired to design those items, so the work is done for you even if you don't want to buy the pattern.

Here are some other ideas.

I would want the wall color to compliment the furniture and other colors going on in the room. What is your color palette?

Look in magizines! Find color suggestions on google. Look on Pinterest. HGTV too.

It depends on the style you are going for. I like simple, clean, and modern, so our walls and couch are both grey. I have floor to ceiling curtains that are red with a black and white geometric pattern, and on the opposite side of the room I have a red accent wall With black and white photos. The rest of the room is decorated with other bold, bright colors, mostly cobalt blue and red, with silver and other metal accents.