Actually your attributes aren't really attributes in the way in which Zen cart understands the term. They are two physically separate products combined to create a product bundle.
Zen Cart can't handle this at present. Indeed, to do so would require a very different data structure to that currently used. The complexity is that the sale of a particular product bundle (e.g. red shirt + blue shorts)can affect the stock of other product bundles (e.g. white shirt + blue shorts and yellow shirt + blue shorts). Handling this inter-dependancy between product bundles is not part of core code, nor to my knowledge planned to be added.
The best that I can suggest is treating your products as separate products (red shirt, white shirt, yellow shirt and blue shorts) and then linking each shirt to appropriate colored shorts, and visa-versa, via the cross-sell module. Otherwise it would require a ton of custom development.
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