Can you set an option to use the track color specified in the gpx file if one exists. Here's what it looks like:

  <trk>
    <name>1. Lions Bay Access Road</name>
    <extensions>
      <gpxx:TrackExtension xmlns:gpxx="http://www.garmin.com/xmlschemas/GpxExtensions/v3">
          <gpxx:DisplayColor>Black</gpxx:DisplayColor>
      </gpxx:TrackExtension>
    </extensions>
    <trkseg>
      ....
    </trkseg>
  </trk>

The color names used by Mapsource and BaseCamp are:

Default
Black
Dark Red
Dark Green
Dark Yellow
Dark Blue
Dark Magenta
Dark Cyan
Light Gray
Dark Gray
Red
Green
Yellow
Blue
Magenta
Cyan
White
Transparent