Soap and lather colour


Forum GOD!
What's the relationship between soap colour and lather colour?

Up until a few months ago, all the soaps I'd used were pretty light coloured, generally white or creamy with maybe only Palmolive Stick being light green. Now I've got The Goodfella's Smile Abysso which is dark grey and I used Extro Cosmesi Arzachena this morning which is dark green, but the lather's still more or less white (a tiny bit grey with Abysso). Had never really thought about it before but what's left on the puck after picking up soap is white too, so what causes that to happen?