This markup allows to set in red the text between <c> and </c>. For example, with

(c3) ab(gh) <c>cd</c>(gh) ef(gh)

the text of the second syllable will be coloured. By default, the color is the same as the red lines, but you can change it by redefining the gregoriocolor color (see its definition in gregoriotex.sty).

rev 1285