Harmony - Troop Symmetry

from the options:
-light dragons
-Elrath dragons
-white dragons
-gold dragons (its scales are gold, after all)

the first two are definitely good, but the third needs to be chosen from the last two, and feel free to suggest your own options, maybe something more interesting will come out

Light dragon (Dragon of Light) - White dragon - Elrath Dragon.
Gold is present in the color scheme of both upgrades, and I think it doesn't work as the main one.

Added after 11 minutes
Regarding the alternate upgrade for the elven dragons: there is a rainbow dragon texture in the game's resources, it can be used.