The Wayfinding settings panel gives you three fields for how the drawn route line looks: Line colour, Accessible wayfinding line colour and Line width.
Where these live
Open your map in the editor and go to the Wayfinding tab. The colour and width fields sit just below the step-free toggle. Changes update the live preview as you type, so tune them by eye. For the broader thinking on readable route styling, see Style your route lines.
Line colour
Line colour sets the colour of the normal route line drawn on the map. If you leave it blank a red is used. Pick a colour that contrasts with your dominant artwork so the route is the thing the eye jumps to, not something that blends into your paths or brand fill.
Accessible wayfinding line colour
Accessible wayfinding line colour sets the colour of the route line when a visitor switches directions into step-free (wheelchair) mode. Make it obviously different from the normal Line colour above, so a visitor can tell at a glance which route they are being shown. If you leave it blank a light purple is used. This field only has an effect when step-free is enabled.
Line width
Line width is the thickness of the drawn line. The default is 2. Too thin and the line is lost over detailed artwork; too thick and it hides what is underneath. Nudge it up a little for large, busy maps.