I posted in our main leader/civ speculation thread, but with respect to units, I see three broad categories of coloration:
1. Default coloration (which we likely have been seeing in some antiquity video reveals). These I would describe as the "Roman," "Asian," and "North American" units (possibly "African" and "Middle Eastern") in the lineup, as well as some of the European units in the antiquity/exploration/modern trifectas. I think default
and associated military units all have some red in them to identify them as "military victory" units (we see some workers with some yellow in them in the Confucius reveal trailer).
2. Independent power coloration. I'm not 100% clear on how this will sort out, whether it will be inherent or determined by your diplomatic interactions, but it seems "city-state type" is returning to some extent. We have seen white-and-black and white-and-green units for what could be expansionist and diplomatic IPs (Slavs, Magyars and Funan; and Ghana and Mixtec, respectively), and we can see a "white on blue" unit in the above lineup (scientific). In a subsequent slide, we can see blue again, as well as red and yellow (military and economic respectively, with cultural's purple being the only color we haven't seen). I am speculating that "expansionism" has been a rough consolidation, not just of IPs but civs, between "Maritime" and "Religious" (both of which are different aspects of "expansion" in a way); no idea of "Industrial" is returning yet, I don't think it's necessary when it can be split into Science/Economic in later eras. What I find curious is that the backgrounds of these little diplomatic units suggest that the background is what "defines" the civ, while the colors might determine something else. Unclear how that logic will all sort out.
Also unclear, is to what extent these IP "diplo figures" are, effectively,
two-colors and what that might mean. I have hypotheses but nothing super locked down.
3. Finally, we know that units are colored with civ's own two colors (not just white-and-color), and that those colors are defined by leaders, who are generally (but not always, in the case of Confucius and Amina) using the same colors as civs in VI did (Hatshepsut uses teal-on-sand, Augustus uses yellow-on-purple, Ashoka uses cyan-on-violet). and there is a single color combination we have not seen in the game yet, pointing toward an unrevealed leader: red-on-blue (which we can see in the above images as well). Also note that the designs on the shields may or may not point toward something HRE/Carolingian (although we see Gaulish boar on generic units in antiquity too so take that with a huge grain of salt):
When I noticed this, I went back and looked through the twitter reveals, and while a good deal of units we see are using their default palette, the unique unit promo videos reveal units which seem to be using the colors of prior Maya and Mongolian leaders:
So, based on new information we saw in this panel, I think it quite reasonable to speculate that a Mayan (maybe Babylonian) and Mongolian leader are planned, as well as a red-on-blue leader (which would track with Norway, but as pointed out elsewhere could be Norman).