Coolant scam

I recently had a new expansion tank put on with a coolant flush done by a certified master bmw mechanic and we clearly established that certified bmw coolant was going to be used . . I checked the expansion tank a few days later and it looks clearly like there's only water in there from the residue along the top and sides. Im sure everyone knows given the design of the expansion tank its hard to tell but what color should I be looking for, Blue or green? and am I really just looking at condensation or is that not the case?



THX

Coolant scam