Just look at the traistone.
Arcane traistones are the combination of 2 colors, sometimes twice the same color.
The colors on the traitstones correspond to the mana used by the spell of the units it can upgrade.
If it has the same color twice, it will upgrade units that use that color, and that color only.
This week’s Arcane Traistone has the green color twice, so it upgrade the troops that charge their spells on green color only.
For now, only 6 common troops uses green color only :
- Glade Warden
- Dire Wolf
(As common cards, they only take 1 Arcane traitstone to get their 3rd trait)
I also expect “Spring Imp” to be released in one or two months (a legendary from “Primal” Kingdom that will probably use green mana color). This one will cost more than one arcane traitstone but will probably not be a good card looking at the other imps (wich I don’t like).