Is there an easy way to obtain a list of uniqye custom fields value from a specified taxonomy value?

For example, I have many categories under taxonomy “products”. Every product has some custom fields, for example: “color”, that I use to build a tag with a list of available product colors.

When i select a product category (f. ex. “Cars”), I wish to filter out the colors that are NOT included in the selected category. In other words, if NO car is brown, “Brown” (though it is a value that is present in other product categories) should not be listed.

The same thing should happen when I select another custom field value (f. ex., “year” on “2010”. If no product of year 2010 is “red”, “red” should be filtered out).

Any suggestions?