IIUC, the icon color should reflect max(casual rank, MM rank).
Now, there are cases when players are unranked in MM. It seems that then their icon color doesn't update despite the fact that they may have a high casual rating.
They have full rank bar on their homepage with the message "rank up on the next victory".
Example,
Kole has passed 1900 casual rank in Sep 2019 and since then fluctuates between 1900 and 2000, as for now Ladder position: 142. Top 7.8%. But his icon is Red Dwarf color. Shouldn't he be Blue White?