This command lets you customize the background color of tracks in the Track view. It's only accessible from the toolbar, and only if the Show Track Colors option is enabled. To assign a custom color to the selected tracks, left-click the paint bucket icon in the toolbar. A pop-up menu is displayed. Now select a color swatch by left-clicking it, or if the desired color isn't available, left-click "More Colors" for an extended palette. To restore the default color to the selected tracks, select "Automatic."
If the paint bucket icon is disabled, it's because either no tracks are selected, or the Show Track Colors option is false.