Menus to filter category archive page by other category or tag?

Hi, I need to add drop-down menus to a category archive page that allow the user to filter the posts in the category by another category.

Here’s an example. I have an archive page for posts in the category “Stationery”. I want a “Color” menu on this page with the choices “Green” and “Red”. When you choose “Red” from the menu, the results should be in the category “Stationery” and the category “Red”.

On this site, “Red” is already an existing category with posts in it. So ideally, we wouldn’t create a new category for “Stationery Colors” and a subcategory called “Red Stationery”. If this is the only way to do it, though, we could do that.

I’ve done this before using a custom post type with custom taxonomies, and it worked perfectly. I’m stumped as to how to do it with normal posts, though. Any hints would be great!

(Filtering the posts by tags or by custom fields would also work. This site has several labeling systems set up already, it’s just a question of my figuring out how to get the categories/tags/custom field names into menus on the archive page!)

Thanks!