Outline items can have a solid coloured background. The attribute $OutlineBackgroundColor lets the user specify a colour (named colour or hex value). The colour then forms the background colour of that note's listing in Outline view; it does not run full width (rather like Finder item colouring) but does include check-boxes and column display if those features are turned on.
A selected item focus highlight uses a tint of $Color. The highlight is drawn on top of the outline background colour if any. These aspects are shown in the image where the first two and last items are selected.
The image also shows that thought needs to be given to contrast between $Color and $OutlineBackgroundColor or titles may become hard to read.