Why Is My Green Anole Always Brown?

There could be any number of reasons, the first being... Are you sure she's a green anole and not a brown anole, which is identical except for the fact that they don't ever turn green.  Was she green when you bought her?  If you've never seen her green then she may not actually be a green anole.

My second best guess would be lack of UV light.  Does she have a full spectrum light to bask under, and is it set at an appropriate distance from her basking sites.  Sometimes, especially if they're wild caught, it helps if they have a window to look out of, or the cage is up against some good sized potted plants or trees to provide more security and the illusion that their not actually confined. If you're sure you have both of those things right, and she's actively capturing nutrition loaded insects and getting supplemental calcium.  Then I'd say you're probably right and she's may just be lonely... But what she's really wants is a male.  I'm not sure adding just another female alone will be motivation enough to get her to want to look her best .  A large tank (20+ gal.) can easily house a couple more, so I'd go for a male and another female... That way she has some competition and more incentive to show her true colors.