Wahoo was also asked to allow contrast control for the turn-by-turn white text on black background, which is hard to read wearing sunglasses. They said no plan for that. I went back to Garmin as a result
We should rather ask for a feature allowing us to customise it because not everybody wants a stronger contrasted line. For mountain bikers it's rather a transparent line that we need to allow us recognise the kind of way we are on or have to follow later in case of rerouting. Why: - GPS is not always accurate, it becomes harder when several ways cross and are parallel, it helps for correct landmarks. - Myself as an OSM contributor it allows us to check easily if the map is correct. - Also imagine you asked for a red line as routing course, some users might have set closed roads or construction roads in red on their customised map.
The Garmin screens are not very contrasty to begin with. That's likely due to the screens being transreflective (which lets them be readable in sunlight without a backlight ).
You can get a better sense of that by using the screen capture option on the units and displaying the image on a computer screen.
