You would not expect 255,255,255 as that is saturated white and leaves no room for variation in the image. The sample will generally make the image darker, so after calibration you would expect it to be a neutral colour and quite bright, but not near 255 on each channel.
Your example does look to be uniform and a reasonably neutral colour. On my screen it seems more a beige tint than green. Which version of the server are you using? The version number is recorded in the about tab. If you are not already on v2.10.0b1 and are happy to use a beta release, then instructions for installing v2.10.0b1 are here. The usual beta release caveats apply, but it has been out for a while and appears to be stable. It includes a much improved auto calibration routine for the camera.