The logos...

[Lufenium]

For references:

English US/UM logos: https://cdn.bulbagarden.net/upload/0/07/Ultra_Sun_and_Ultra_Moon_logos.png

JPN US/UM logos:
https://cdn.bulbagarden.net/upload/a/a9/Pokémon_Ultra_Sun_logo_JP.png
https://cdn.bulbagarden.net/upload/4/4a/Pokémon_Ultra_Moon_logo_JP.png


In the countdown to Sun & Moon, the diamond that was found in the Non-English logos turned out to be the Z-Crystal (lol the fusion rumours). In the JPN US/UM logos, replacing that golden diamond is a multi-colour triangle shape that is somewhat reminiscent of the Island Trial Amulet. https://cdn.bulbagarden.net/upload/1/13/Key_island_challenge_amulet_Sprite.png

It's obviously going to be of some importance, like the helix symbols in the JPN X/Y logo and the diamond in the JPM S/M logo, so what do you think this could be?

 

[Like_Fahrenheit]

It's also Necrozma's face (to a point). The 7 colors of the rainbow or in this case - visible light spectrum (Roy G Biv). Seeing as Necrozma is clearly a part of Solgaleo and Lunala this time I think this triangle is the item used to change their forms/fuse or something similar. In short, it's to signify the new mascots.

 

[PurplePopplio]

It's going to probably be Ultra Forms/Evolution/Fusion.

 

[PinkIceFox]

It's the symbol on the forehead of NecrozmaXSolgaleo/Lunala. So it's basically the symbol that shows Necrozma is controlling them or whatever, probably.

 

[Astro Pantherion]

Likely the following is very unlikely, but maybe the colors of the triangle may represent the various "formes" that Necrozma can take? Someone already speculated the idea that the Solgaleo and Lunala we see are more like energy constructs rather than the true legends of the original games with Necrozma mimicking them by building identical bodies for itself, meaning if true, the Legendaries are Necrozma Formes. Both are also one solid color, being yellow and cyan respectively.

Maybe each color has a different setup of how Necrozma shapes/rebuilds itself to appear differently, as its pretty much living armor.
1. Yellow - Solgaleo-like form (Lion mode - Psychic/Fire as early rumors claim)
2. Cyan - Lunala-like form (Bat mode - Psychic/Dark as early rumors claim)
3. Red - ????
4. Blue - ????
5. Orange - ????
6. Green - ????
7. Purple - ???? (maybe a dragon-like form, being Psychic/Dragon?)

Again, this is just pure speculation with no real proof pointing/supporting this idea.