How do you make brown paint grey?

Simple solution:

In painting, you would select a brown and gray close to the shades you are looking for. Start with the lighter shade, and add a small amount of the darker color. Mix the two colors, and then add some more of the darker shade. Continue to do this until you get the color you are looking for.

More advanced solution:

You can mix your own gray using black and white. To get your brown mix orange and green. To have even more control over the shade of brown you achieve, mix red, yellow, and green in small amounts, adjusting for the shade you want as you go. White can also be added to lighten things up a bit if needed. Once you achieve the desired brown and gray, you then mix them using the step above in simple solution.