In the building tab, there are more "sub-tabs". The second last one should have an icon of 3 circles, RBG. Select the tab. There should be a "block" that has the same three circles with the words BG under it. Place it where you want to change the colour and click edit object. The rest is self explanatory enjoy.
Click to play:
geometry dash l b612 l retrica
Or you have to use the Background color tool. It can be found in the last section in the items list. It activates the color changing the instant the player goes by it, it does not have to make contact with the object itself and it isn't visible in-game.