TackTracker does pick a random color for each boat, but as you say, sometimes this does not achieve the best result. If you have a Club Regatta license (on a PC), it is possible to have TackTracker choose the same color for each boat.
This is what you do:
This will enter the competitor data into a Competitor Schedule file. You can see the file as the first entry (at the top) of your library. Now when you load the tracks for subsequent races, it will use the Competitor Schedule data to select the correct color (and other details too).