I don't see them changing any of the other ones (except for slightly different color shades from the original).
-The first American 2nd edition volume had the color and picture change from the original in order not to confuse people with the
Best of Red book (although I'm not sure why they didn't change either the picture
or the color instead of both)
-The second volume kept the original picture but changed the color for some reason. I guess they felt that red was a better color to go after blue than green was.
-The third volume kept the original picture but changed the color because it was the remaining color out of red, green, and blue left. The first volume took blue, the second took red, so green was left. They must've wanted to keep it like the Japanese version in the sense that there'd be a red cover, a green cover, and a blue cover. (Just... in different orders.
-The fourth volume kept the yellow cover but changed the picture in order not to confuse people with
Best of Yellow. (Although the 1st edition release of the American fourth volume uses the same picture as the Japanese cover and
Best of Yellow, but with a different background color...)
-The fifth volume (unless they change it at the last moment) keeps the reddish-orangish cover and the original picture.
-The sixth volume (unless they change it at the last moment) keeps the green cover and the original picture.
And so... all Viz did is change the pictures of the two volumes that would otherwise look too similar to the
Best of Books. And they switched the red-green-blue color order of the first three volumes. I don't see Viz changing the background colors or the pictures anymore for future volumes, considering that the fifth and sixth volumes so far have not had the background colors or the pictures of the characters changed.