I have dealt with this and I think there are a few questions to ask.
1. Do the different color variations provide keyword opportunities. for example, are people searching for nice-shirt-blue and nice-shirt-red, or are they just searching for nice-shirt?
2. If it does not provide any sort of search benefit to separate the shirts, then you may consider combining them into one product page. You may consider providing an image with an array of the products on your overview page to show that there are multiple colors within the product page. This will ensure that any links built to this product are not spread over multiple product pages. Also, from a usability standpoint, is it likely easier for a customer to click through images on one page to decide the color that they want rather than to click the back button to find the new product.
3. Finally, if you decide that it is better for your customer's experience to keep the products on two separate pages, but the color don't provide any keyword or search benefits, you may consider choosing one color (say blue) and setting a rel=canonical on the other colour variations to the blue shirt page.
Ultimately, it boils down to "what is the best for your customer". Search engines ultimately care about user experience, so if you are able to create the best experience first, then decide whether unique descriptions, rel=canonical, or configured pages are best for you.
Hope this helps!