GGIS: what line styles work best with Garmin maps

I am using QGIS to create some maps to load on to a GPS using the plugin. When I display the maps on the GPS the lines are very faint and I can not distinguish colours and some colours vanish altogether. But even the black lines don't stand out.

I have tried increasing the thickness of the lines in QGIS but this does not seem to make any difference.

I am displaying the maps on an ETrex 30.