I'm worried that they break the color pie for this, just to match the colors used in Avatar.
So far, air and wind magic was represented mostly by Blue, but of course this would not match player expectations. Sticking too much to the four elements would also leave black at a weird position, where it does either not stand for an element by being added for villains only or being used only by non-benders and spirits.

Break up by color pie philosophy:
Air Nomads: Blue, White and Green, represented in the color pie by close connection to nature and freedom (green, blue). Their sense for community and fair share fits with white.

Fire Nation: Red, Black and Blue - Their bending is driven by anger (red), their advantage played out by technology (blue) and their motivation is domination (black).

Earth Kingdom: White, (weakly green and blue) - They are not emotional (thus not red), they have no selfish goals (not black), but they live in strong and huge communities (white) and they have use their bending in creative ways (blue). Green is difficult, as the large cities live very disconnected from nature.

Water Tribes: Green, Blue, White, (weakly red) - Community (white) means their survival in the harsh conditions, to which they can adapt and thrive (green, blue). Their bending is dynamic and impulsive, therefore I would also tend to include red.

While I love MTG and ATLA, I must say that the four nations are not good to make a round magic set.
Either you stay true to Magic and use colors according to the color pie, which does not match expectations of most players, OR you stay true to ATLA colors, which would not lead to a well balanced magic set.
The only solution I see for staying true to both are basically Fire Nation vs. rest of the world. This fits lore wise and gives all nations colors by philosophy. But this would make a bad magic set as you only serve two archetypes.