The brown zooxanthellae coloration overpower the base blue coloration of the anemone. He is much heathier right now and he will stay like he is now.
To improve his coloration, you can provide more intense light, and decrease the disolve nutrients of the tank. If you can do this then the zooxanthellae population will decrease and the blue coloration will be more intense. This will bring the coloration of the anemone out more.