This happens to the game source when I tick "use colour blend", which blends two colours from top to bottom of the screen as the background including the margins. Under "game", under "attributes", there is an attribute named "colourblend" with the value "true", then, there is an attribute named "colourblendtop" with a value that is a "string" which is a colour name, as well as an attribute named "colourblendbottom" with a value that is a string which is another colour name.
How can I use a script to change the top colour or bottom colour or both during the game at some point (when entering a room or as part of a verb response)?

game.colourblendtop="Black"
game.colourblendbottom="White"
JS.colourBlend (game.colourblendtop, game.colourblendbottom)

or change it directly

JS.colourBlend ("Black", "White")