Powdering

Copper sulphate – CuSO₄ – is bright blue, though copper is pink, sulphur is yellow, and oxygen is colourless.  If you grind up copper sulphate it will go pale, and eventually it will look almost white.  This happens because at a transparent surface, some light is reflected.  If there are thousands of surfaces, very little of the light gets into the copper sulphate.  A similar thing happens with a pile of glass panes – you can’t see through it because it reflects most of the light.  This is not an intuitive result – each surface reflects only about four percent of the light if it is clean, and yet the result of many surfaces is that very little light gets through to the other side.  Headlights reflect well from fog, rather than penetrating it.