It’s a ceramic coating. Reflects heat to the rim.
More likely keeping brake heat away from the wheel
Hadn't noticed it before, but their DRS actuator is similar to Ferrari's this year (integral swan's neck), and it was there in testing as well. Interesting parallel. Last year they both introduced shrouded mirrors at the same time as well, correct?
It seems that Toro Rosso is still using AppTech wheels.