Get available taxonomies from taxonomy

Hi there,

I am running into an issue with selecting taxonomies;

Basically I want to have a list of available options upon a selection of either;

taxonomy 1; color
taxonomy 2; size

Upon selecting a color I want to show the user a list with the available sizes with that color.

Right now I select all posts with that color, and create a list with all sizes available. Though I am sure there is a better and faster way.

I can not seem to figure out a faster way of getting the info / the correct syntax.

Hope anyone can help me!

Thanks,
Matt