Merged TMA track doesn't observer specified colors for track/infills

[IanMayo]

The preferences pane allows the analyst to specify what colors should be used for merged tracks. One color for the merged track points and another for infill segments:

Steps to reproduce:

• open mid_flow.dpf
• expand Track:TMA_1441
• expand Positions
• Split positions into multiple legs, deleting points to give space for infills

TMA track should look like this:

• Select TMA legs
• Right click and select Merge track segments - into new track (highlight infills)
• Note that the while the infills are green (as specified in prefs) track positions are pink, instead of red:
  .

Note: the default value for track positions was pink - so maybe we're reading that value, instead of the current one.