I noticed in blender than when I try to use full white (as world color or material color) it shows as a gray color. I confirmed this by taking a screenshot and using the color-picker tool in Photoshop.
However, in blender 2.76 white is rendered as actual white.
The settings you change via the preferences and the saving of the startup .blend are reset whenever a new release comes out. The issue might be because of settings that were reset and not due to a bug.
I replaced 2.77’s startup and preferences files with 2.66’s and it fixed the issue. Never had this problem when upgrading with previous versions, but I guess I’ll keep copying them over for new releases if I get the issue again.