Hi joanwang
Link to start of thread in old SE message board for those that may want to read full discussion -
Contamination in TBC bond coat
Assume all spray conditions/parameters are constant, except coating/substrate temperature. Many TBC coatings will show a different colour depending on this temperature. An old guide to some coatings was "antique white" with "grey/white" indicating too low coating/substrate temperature and "orange/yellow" indicating too high. To be honest, I'm not sure if this is very helpful in itself - except in the case where a colour
change is noticed say during production spraying, as this will be a sensitive indicator of changing process conditions and should be investigated.