Curtain Colour Advice?

Since everything else is neutral, I would pick curtains that pop. Maybe a rich contrasting color or an interesting pattern that ties in the shades of brown in the room. If you are buying curtains, get some you like and try them out. You can always return them if you don't like them. If you are making them yourself, get a small swatch of the fabric to take home and try against the walls and furniture to see if you like it before committing to buying yards and yards of fabric.

If you have pets, I can tell you that darker curtains will hide fur and marks from them peeking between the curtains better than light curtains.

Also think about curtain weight. If your window gets direct sunlight, you may want heavier, darker curtains to block the sunlight in the summer. If your living room window is in an area that is shaded, you may want lighter more airy curtains that will let some light in.