The easiest way to start is to find a fabric item (pillows, rug, curtains) that contains your new color (gray) and your old colors (brown) together in a pattern. Then use that gray elsewhere in the space to create some flow.

You did not mention if you have an area rug. If not find a carpet with grey, brown and beige and a soft yellow. Then paint the walls a very creamy light yellow.

But if you are not fond of that color pallet you could go towards blues.

W Brown has good advice.

Right now, our walls are a warm gold. Our area rug is beige. We wanted to paint the walls blue to give the room a different look so I'm happy to hear you say that!