So I create a 100% white frame in Marquee, to drop into the timeline and use as a background.
It displays as slightly grey when cut into the timeline. I have to use Color Correction to raise
I have experienced this on two systems, at work and at home.
Both running MC 6.
In SD and 1080 HD.
Title Tool, and most likely Marquee legalize levels into video specs when saving - so what would be full white 255 RGB is now 235 RGB.
Media Composer is a Rec/601/709 system. You can cheat it and work full wing with imports, and now LUTs to go full scale, but there are still operations like titles and some VFX renders that will make legal video.
In your case, create the white frame outside of Media Composer and import with 601/709 levels so that Media Composer leaves it alone.