This tutorial explains how to change the backdrop colour of a game in WoofJS (so you can use a colour instead of an image for the backdrop). Create a new project at woofjs.com and add the following code. Make sure you pay attention to the comments in the code that explain how it works.
// You can set a backdrop colour using a hexadecimal colour code
// Step 1 - Go to https://htmlcolorcodes.com
// Step 2 - Select a colour
// Step 3 - Copy the 'HEX' code including the #
// Here is an example for a light green background...
setBackdropColor("#76F67E")
// PS. You have to spell color the American way in setBackdropColor
// Hexadecimal (hex) is a number system that has 16 digits
// These digits are numbers 0 to 9 and letters A to F.
// Hex colour codes are six characters long. The hex system
// allows us to create short codes for millions of different colours.