white solid not white in render - help

I have video with a white background. I move it left on my screen, leaving a black gap on the right side of the frame. I create a "solid" that is 255,255,255 white - as white as I can get it.

When I look at it in the preview in Ppro cs6 - it looks perfectly white. I can't see the difference in the background.

When I render - the white solid isn't white - it is an off white.

see http://12z.us/test.mp4

same issue if render in low resolution or very high resolution.

how can I fix this? thanks!