The following code produces an "Insufficient Text Color Contrast With the Background" error even if there is no contrast issue:
<p style="background-color: transparent;">
Removing the background-color styling gets rid of the error but it's impossible to see it in the Rich Content Editor. UDOIT cannot change the color using the arrows to adjust value up and down in the UFIXIT editor (nothing happens if you use it), but it can successfully change the background color using the color picker drop-down.