When we started to work with Robeez back in 2001, Sandra thought she sold one product: baby shoes. The reality was that she had about 30 different styles. Most were listed individually so all the shoppers had to do was selecct the size. The exception were her "classics", solid colour shoes which were listed as a single style that had both a colour and size selection. While the other styles were shown with individual product pictures, classics were shown in a group photo. It reminded me of the old Sesame Street rhyme "one of these things is not like the other.." It was easy to convince her to separate them into individual listings so that all of the photos and shopping controls matched.
Since then, we have come accross this issue dozens of times. Since our main market is small to medium manufacturers and retailers, most of our clients don't have huge catalogs so I usually recommend showing different colour variants as separate items. There are many reasons for this:
1) Grow your catalog: We could have built Robeez with a single product and had shoppers choose style and size, but that would have been anti-climatic. Creating each item as a separate product got more product in front of site visitors and made their visits more enjoyable.
2) Increase content: Search engines index each page so the more product pages you have, the more pages you have for people to find.
3) SEO: You can tweak your product descriptions, titles, and meta-description to focus on different keywords. You can also increase your chances if someone is looking for a "Blue Men's Cotton Shirt" if you have a page that lists your "Blue Men's 100% Cotton Shirt", not "Men's 100% Cotton Shirt, comes in 6 colors"
4) Shopping Portals: These allow you to list all of your products, display pictures, titles, description and links into your catalog. You will have a more effective presence if you list each of your variants separately. If someone is searching for a blue shirt, you will show them a blue shirt, not a green shirt that also comes in blue. Which link would you click?
Think strategically about your catalog. It is worth the effort.