The traitstone’s colors match up with the troops that can use it. This can be followed for all arcane traitstones - the two dots on the picture display the color combination. Arcane Stealth is mono-green - only for mono-green troops - and the only mono-colored troops are commons.
Dire Wolf
Glade Warden
Goblin
Reaver
Satyr
Tauros
Due to the low costs of those, you’d only need six total (one each for their last traits).
Of course, more could show eventually or a conversion system might be introduced, so its not completely wasteful to have an extra or two.
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
Reaver
Goblin
Satyr
Tauros
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).
Don’t have the maths off the top of my head. But Spring Imp is due around first, maybe second, Monday of February. Which incidentally this year is the 1st of Feb.