First, I'm just going to post a comparison of how the Muton Elite's look can vary, depending on the environmental lightning. On the left side image it is during Day, while on the 2nd it is during Sunset/Sunrise and with a fire closeby.
As you can see, the enviromental light has turned the blue into a near purple (at least for me, since color perception can also vary between people). With the Muton Elite its color isnt' really an issue, since it has plenty of other visual clues to help identify it, like its unique look, helmet, weapon, etc.
But what about the ADVENT hybrid units?
This is the ADVENT Sergeant on the 7 different types of background lightning that can appear during the game (Day, Sunrise, Sunset, Vortex, UrbanNight, Rain, Aerial, etc.). To me, while during day (1st image on the sequence) the blue is clearly visible, but there are other situations where it nearly disappears or becomes more enhanced, turns into a near green, etc.
And this range of appearances means to me that in certain lightning situations the colors either become useless to distinguish between different ADVENT hybrid units, or they just confusing. In one mission the Sergeant will 'look' green, while on the others it will be blue, and meanwhile the ADVENT First Sergeant will appear on missions, and it is a 'green' colored unit (which will then change colors depending on the lightning).
Finally, as I said above, color perception can vary between people (and there are also those colorblind, for whom certain colors are impossible to distinguish from others), so this might not be an issue for most. But for me I think a simpler color scheme would be better for the ADVENT hybrid units (Gunner, Grenadier, etc.) both for aesthetics and visual identification.