Ubercart

I am setting up a site that sells electronic parts and came across a similar issue. For example, there are 160 different values of resistors that I sell in 2 different power ratings. The problem is that I do not yet have available all of the values in the higher power. This is like your medium green bag problem. I ended up abandoning attributes entirely and creating a separate product for each value in each power rating. This seems to defeat the purpose of attributes but I could not see a good way to use attributes as they currently exist that would handle this situation.

It currently isn't possible (that I know of) to tie one attribute option to a different attributes option, but if what you're after is only what you've stated above the best way to do it would be to merge the options. For example, instead of having an attribute for size containing 'large' and 'medium', and an attribute for colour containing 'green' and 'blue', you would have a single attribute (named whatever you'd like) with the options 'large green' and 'medium blue'. At least that's how I'd do it.

Unfortunately if your attributes/options get much more complicated than the above it would be quite messy. If someone were ever to create a contributed module for attribute rules I fear it would be quite complicated.