Created attachment 180814 [details] The process of dual-colored lines now... An enhancement to line attribute- line types of dash / dot-dash where the "unfilled" or background color of the line is given a secondary color. In electrical work (especially automotive) wires with a main color and a stripe are drawn in schematics with 2-color lines. These show up as dashed lines of 2 colors.
When I have to do this, I simply create a copy of the dashed line, change its style to solid and change the color. Then I align them to be perfectly on top of each other. This is a bit cumbersome, I know. To implement what you're proposing we would have to: 1) Add an option for background line color (which does not exist ATM) 2) Change the dashed line drawing method I could not find an equivalent feature in other office suites. Let's hear from the UX team about this idea.
With the line style you define the on/off pattern different from solid. Could be dash-dash but also dash-dot-dash. This makes it more clear that no fine-grained line definition is possible, such as style of dashes, style of undashes, of dots of long dash... As Rafael commented: draw a solid line in the background or use a copy shifted by the dash width.