onHwb method
Creates chalk with background color defined from HWB (Hue, Whiteness and Blackness) color space parameters. https://en.wikipedia.org/wiki/HWB_color_model
Implementation
Chalk onHwb(num hue, num whiteness, num blackness) {
var rgb = ColorUtils.hwb2rgb(hue, whiteness, blackness);
return makeRGBChalk(rgb[0], rgb[1], rgb[2], bg: true);
}