Sorcha,
the green page is the same as the one with the celtic knots.
It has a body with a background color of 00ff80 which is a slightly bluish green. It also has a background image defined (the white background with the celtic knots). The browser sees the green background command first, so it makes the green background. Then it takes some time till the background image is loaded, and as soon as it is loaded it replaces the green background.
When you go back to that page, the background image is in your browser's cache and thus will be displayed immediately. So on your way back you won't see the "green" page.
MudGuard