Categories in reverse

[onjegolders]

Am sure this has cropped up loads but as it's been a while since I used categories I'm struggling to remember how it all fits together.

I'm trying to list all categories that have been linked to by at least one project.

My page field links a project to a category and not the other way around so I'm struggling to come up with the right code.

Home

About

Projects

-- Project 1 (green, red)

-- Project 2 (blue)

Contact

Categories

-- green

-- red

-- blue

-- black

-- yellow

In this example on my work page, I'd only want to output filter links to green, red and blue.

Am I missing something very simple? 

Probably

[diogo]

I knew I had answered to this before! It wasn't easy but I found it

http://processwire.com/talk/topic/1916-another-categories-question/

[onjegolders]

OK, so you're already a legend so I can't give you much more praise than that

[diogo]