What does foam and color say about water quality?

Hello,

if I see foam floating around in water, is this a sign for bad quality (example: http://img35.imageshack.us/img35/4707/p7190183.jpg )? Also e.g. in Sweden I took water from a very good looking river which was floating fast and looked safe to drink, but the water had a yellow color. Until now, I think that color doesn't matter that much, but I'm caution with foam. But who knows for sure and can tell what color and foam means for water quality and why they are there at all?

Thanks in advance,

Björn