[NTG-context] Colors in lmt_surface and luametafun

Hi Hans, On page 36 of luametafun.pdf there is the following example of using lmt_surface: \startMPcode{doublefun} draw lmt_surface [ preamble = "local sin, cos = math.sin, math.cos", code = "sin(x*x) - cos(y*y)", color = "f, f/2, 1-f", color = "f, f, 0", xstep = .02, ystep = .02, xvector = {-0.4, -0.4}, height = 10cm, lines = true, ] xsized .TextWidth ; \stopMPcode What is the meaning of « f » in « color = "f, f/5, 1-f" »? Is it possible to specify other colors and how can it be done? Thanks for your attention: Otared