Just played a match with NB1 where cryptex converted to white gems.
As far as I can determine the following happened.
I play NB1 against Elspeth in the PVE “Duets of Mana - Spite and martyrdom”
Elspeth greatest mana bonus is 6 in white while NB1 has a max bonus of 3. So I reckon that cryptex finds the highest mana bonus and converts to that colour, instead of my highest mana bonus.