You list colors so that the number of colors in between them is the smallest, reading WUBRG left to right, cycling back to white once you get to green.
Look at literally any green and blue card or any red and white card. Coatl is 1GU, not 1UG. Lightning Helix is RW, not WR.
Green blue decks are generally referred to by the abbreviation UG because of UG madness, though that deck in and of itself made a mistake in its name.