[NTG-context] how to apply gradient color to a piece of text?

hi all, when i look at luametafun manual there are some intresting examples that can apply gradient colors to metapost paths. how i can apply this cool feature to a piece of text? i tried the code below but it does not produce any result. \definecolor[Gleft] [h=9f9393] \definecolor[Gright] [h=ff1aa4] \startMPpage picture Mytext; Mytext := btex hello etex; draw lmt_shade [ picture="Mytext", direction = "right", domain= { 0, 2 }, colors= { "headerGleft", "headerGright" }, ] ; \stopMPpage