1936 11 228 Retroflexion is..interpreted liberally to mean any sort of ‘r-coloring’ of the vowel.
1950 D. Jones xvi. 82 r-colouring, when vowels are said with simultaneous lowering of the soft palate.
2004 W. B. McGregor iv. 85 It is usually the quality of the preceding..vowel that distinguishes these consonants: it usually has an r-like quality called r-colouring near retroflexes.