Screen resolution change during the intermission screen makes the background white

Self explanatory.

Did you get the Emerald token?

It seems that SRB2 takes a picture of the scene ahead of you for a background when you finish the level, but when you change resolutions, I'm thinking that it's resetting the video and the background in the process.

If this is so, then it would probably be best left white, unless you want a weirdly drawn background afterwards.