In Millon's test, first the compound (to be tested) ie, protein is nitrated by $\mathrm{HNO}_{3}$ and then complexed with $\mathrm{Hg}(\mathrm{I})$ and $\mathrm{Hg}(\mathrm{II})$ ions. A red precipitate is obtained. However, if the protein contains tyrosine, sometimes initially a white precipitate is formed that turns red on heating.