Function material_band 

pub fn material_band(
    colors: &Colors,
    ground: AlphaColor<Srgb>,
) -> (AlphaColor<Srgb>, AlphaColor<Srgb>)

The darkest and the lightest colour a translucent surface of ground shows, whatever lies below it: on the side of the ground its ink stands on, the ground stepped toward that end of the theme’s neutral ramp until it stands material_spread off it (or reaches the end); on the other side, the ramp’s end itself. The backdrop pass compresses what shows through into this band before the tint is laid over it (crate::backdrop::Legible), so these are the worst backdrops ink on the material is measured against.