Never heard of that before. Are you sure that you are using the same versions? Is this a new game or a game saved in previous versions?
Try having both you and your opponent set your dice colours again. Even if it appears to be set on the drop down, actually click on the list and choose a colour.
Make sure you are both setting the dice colours on the Chat tab in Preferences. It could be that one of you is setting colours for Dice Over the Map, which show differently.
Maybe ask your opponent to to do screen grab.
Plus, upgrade to 6.6.7 and VASSAL3.7.5 as per my previous.
Doug