Did you just get the damsel? Looking at the time of your post, was this his color at night with the lights off? If so, did he go back to normal color shortly after the lights came back on?
I believe Damsels will turn colors at night as some sort of protection, i.e. blending in with the night.
If this isn't the case, you probably should leave more information for us to help, info like how long your tank has been running, what size, lr or ls, ........
HTH