Double-click the label in the Audience column. It will open a window that will allow you to modify the color. There may be a method to do this for many groups or users at one time, but I couldn't find it. I ended up just doing it individually for the dozen-ish groups that were dark blue on the black background. Now they are light blue or teal or something, and much easier to read.
I did have UniTrunker crash and shut down once when changing the foreground (text) color for a group, but I could not reproduce the effect.
I've learned that changing the default colors for service groups, such as police, only affects new imports or entries, not existing entries.
Also, If you want to change the color for multiple existing entries, do this:
Highlight the receiver being used, in my case an in-line receiver, BCD396XT;
Click the "Systems" tab at the bottom;
Highlight the system you want to make changes to;
Click the "Inspect (magnifying glass)" icon in the top bar:
Click on the "Groups" tab in the window that opens;
Use the standard Windows' select options to highlight the groups whose colors you want to change:
Click on the "Inspect" icon;
Make your change in the "Color" line, using the drop-down menu if desired:
Click the "Okay" button in the "Color" window;
Voila!