You have a room full of neutrals, so you really have a lot of options. The aqua you suggested would look great. Red and aqua also pair well together, so you could bring in some red accents with your aqua as well. Red, orange, yellow, chartreuse could also work as accent chairs with your brown and grey furniture. Unless you want your room to be really dark, just be careful not to go too dark with your wall color, rug (just dark enough, so it's not too easily stained) and other finishes and accessories.
For rugs, I prefer a short pile wool blend, they're soft and relatively easy to keep clean (as long as they're not too light and have a pattern). It's really a personal preference (I just don't generally care for the look or feel of sisal). Only after 11 years in a heavily-trafficked area is my wool rug finally looking worn. Not bad.

Patterned. Not only will it help tie together whatever accents you decide to add, it will be much more forgiving stain-wise. My oriental rugs have hidden a world of sins.

Thenmake your chair color and accent color decisions based on the rug you find that works with your other major pieces. It doesn't have to have both brown & gray, but I'd try to find a rug that has at least one of the two colors in it.

Like your aqua idea - look at how many rooms Candace Olson has done successfully with aqua & brown or gray! KB has some good ideas too.