Please follow the below instructions to assign/add colours to your backboards:
Navigate to the Sign Customiser admin on your store.
Click on "Customisers" in the home menu
Click on the customiser you wish to edit.
In the left-hand side menu click on "Backboard Colors & Materials"
Click on an existing backboard color that you have already created.
Scroll down untill you see "Assign to backboard"
Use the dropdown menu to select the existing backboards that you want this backboard color & material to apply too.
Click on save at the bottom of the form to save.