How are they a creamy coloured top on Guinness stout, which is black?

Answer 1

There is a lot of air in the foam, which makes it appear lighter. The liquid is denser and blocks the light a lot more.

Answer 2

That's the head. A properly poured stout is a good thing. The foam should be 1.5 inch's on the top of the pint.
As you down this portly brew ye may want to say a little ditty like this:
May the good saints protect you,
And bless you today.
And may troubles ignore you,
Each step of the way.

Answer 3

Mmmmmm, Guinness......

The head on beer is always lighter coloured than the beer itself. It has something to do with the gas trapped in the bubbles.

Answer 4

A lower density means a different refractive index.

For more info on beer foam, look here:
http://oz.craftbrewer.org/Library/Methods/Other/KunzeFoam.shtml