There can be a slight chemical reaction by adding milk. Be refined and have a slice of lemon.
there is wiv lemon - try it - goes lighter as a result of the lemon acid. ( the tannin when charged by those leedol H+ ions is less brown)
and do you know, I am SURE that was what gave the fella the idea to seek other more obvious colour changes, such as litmus. Just me
MILK there is discussion somewhere - milk last, the first drops are at 100'C and so the reaction MAY be different to milk first where the first drop of tea only heat the milk up a little