But. We have plenty of kames, formed when a hole melts down through the glacier and the streams flowing into the hole drag gravel along with them. That implies that there was gravel on top of at least part of the glacier. When one goes through a valley, lots of stuff gets ground off the walls of the valley and some winds up on top. However, Kettle Moraine isn't famous for its steep valleys--the whole area was under the ice. So somehow, lots of gravel wound up on top. And if gravel did, surely there was dust and maybe dirt as well.
The Glacier Glossary defines "supraglacial" this way:
"The area on top of the glacier which may be snow, ice, rock fragments or covered with soil, plants or forests."
You've seen what happens to December snowpiles that last into February--white isn't the color most of us would use to describe them. Green I wouldn't expect. But if February lasted long enough...