r-colouring has the acoustic effect of lowering the frequency of the third formant, as can be seen in the diagram below. This is an utterance consisting of the vowel ɜ to which r-colouring is added at about 0.7s on the time scale. The third formant frequency descends from approximately 2.4kHz to approximately 1.6kHz
This effect can be achieved in a number of ways. Perhaps most commonly the vowel is accompanied by the raising and retraction of the tongue tip. Another way is to bunch the back of the tongue.
r-colouring is symbolised either by adding a hook to left of the vowel symbol as in ɝ or by adding a superscript as in ɜʴ
r-colouring is a feature of a number of varieties of North American English. Typically speakers have r-colouring for the vowels ə and ɜ only. For American speakers it usually occurs in words where the vowel is followed by <r> in the spelling. An example is the word further ˈfɝðɚ. Some speakers also have an r-coloured version of ɑ as in card kɑʴd.
Also called rhotacisation.