Category summation

Each post on my site must have 2 categories: a, b or c plus red, black or blue. I want to do something like this:

– If I type mysite.com/a/ I will get these posts:
a-red, a-blue, a-black

– If mysite.com/a/black:
a-black

– If mysite.com/black:
a-black, b-black, c-black

How can I do this? Cuz if I type mysite.com/a/black I get same posts, like in mysite.com/black