Let's continue with the first button on the right of the colors (outlined in red in the screenshot below)
This option ( ) will let you apply a color to the current layer.
On the same prooject, we can, for example, decide that all the background layers will be in the same Red group. The character, as an element of the animation, will stay in Yellow. We will then have the following Timeline and Color group panel (after refhresing) :