It appears that Gaby's RD dress colour is "teal".
The part on the top of her left breast is actually the same fabric but reversed. I'm not 'in' on anything but you can see it clearly when looking at the bottom of the skirt when it moves. (That and I'm an arrogant ***** that is completely convinced of the correctness of her observations.

:laugh[emoji4]
Now we only need to find the correct name for the colour of Guillaume's FD costume. It's not really maroon but it's also not really purple. It's both or in between. Anybody knows a name for that?