Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
purpŭrasco, ĕre, v. inch. n. [purpuro], to grow purple, become of a purple color: unda cum est pulsa remis, purpurascit, Cic. ap. Non. 162, 31 (Ac. Fragm. 2, 7 B. and K.).