View the info palette (F8) and put the cursor over the background.
If the green value is higher than the red and blue values then the colour is green, your monitor is just unable to show it correctly, possibly due to monitor calibration.
If this is the case try running the adobe gamma app to balance your screen.
If that doesn't work, perhaps your colour prefs are messed up, have you changed them? I would usually recommend changing them to a colour space of adobe rgb or apple rgb if they will end up in print.
5.5 default is sRGB, I believe. (I am pretty sure the s stands for Sh|t) and posterises the onscreen appearence of certain colours.
If the rgb values are the same eg 0,0,0 or 5,5,5 you are getting black and this could be caused by jpeg compression. Use a non lossy format such as tiff or psd. (This would seem unlikely though,as someone else is able to view the colour correctly)