Basically phosphors decay at different rates.
CRTs do this too, though many direct views have very slow green phosphors especially to reduce flicker so you're more likely to see green trails that really stay for a while (whole fractions of seconds, which is a long time) as opposed to decay-rainbows.
Projection TVs you will find, especially HD sets and front-projection (designed for graphics) have faster phosphors and should be run at faster refresh rates, but still the phosphors decay at different rates. As you increase the refresh rate, at some point you won't be able to see flicker, because your eyes are too slow. However, flicker is still occuring as the phosphors decay, and because they decay at different rates you get that kind of yellow decay in between refreshes. As you dart your eyes around you may still see this after-effect despite the fact that you can't sense the flicker that is occurring.
I am particularly sensitive to this kind of thing (rainbows on 1-chip DLPs drive me nuts), and it's why I run my CRT projector at 72hz progressive, though I still see some decay artifacts even at this high a refresh rate.
Viewing with ambient light and things like that will vastly reduce visibility of these artifacts, FYI.
Again, most people will never notice these kinds of things, or if they do inadvertently notice them they won't be bothered by them.