calling Darker() on a dark color can return a lighter color

[kmanley]

Great library - thanks for making it. One small thing I noticed is that calling Darker() on a dark RGB color can actually return a lighter RGB color. It would be more intuitive (at least to me!) if it clamped to black. On a related note it would be useful to have helper functions like IsDarker(color1, color2) bool and GetContrast(color1, color2) in the package.

[kmanley]

I fixed this in my local fork by changing Darker to

// Darker returns a darker version of the specified color
func Darker(c color.Color, percent float64) color.Color {
	col, _ := colorful.MakeColor(c)
	h, cv, l := col.Hcl()

	r, g, b := colorful.Hcl(h, cv, l-(l*percent)).Clamped().RGB255()
	return color.RGBA{r, g, b, 1}
}

I think there may be other places in gamut code where Clamped() needs to be replaced by Clamped().RGB255()

[muesli]

Can you provide an example where Darker produces wrong results? I'd love to add a test for this.