What is the easiest / best way to put them back to default?

Thank you.

In the "Tray Icon" --> "Colors" section of the Weather Watcher Live Options window, click the background color boxes and set the colors to the following:

Severe Alert Found
Red = 192, Green = 0, Blue = 0

Temperature Below Freezing
Red = 0, Green = 0, Blue = 128

Temperature Above Freezing
Red = 4, Green = 0, Blue = 0