A question for developers and SEO specialists: should an online store have a separate URL for each color and size?

A ques­tion for devel­op­ers and SEO specialists. Should an online store reload the page when the shop­per picks a color or size? That is, should it have a sep­a­rate URL for every combination?

On the one hand, that’s con­ve­nient for export­ing prod­uct feeds to shop­ping aggregators.

Here’s what Yandex recommends for export­ing prod­ucts in the “Clothing” cat­e­gory to Yandex.Market, Yandex’s price-comparison service:

Offer link on your site

For dif­fer­ent color and size vari­a­tions of one model, it is desir­able to give a sep­a­rate link to your site for each com­bi­na­tion of colors and sizes in the offer, but you can also give the same URL for all variants.

[Yandex goes on to explain that with sep­a­rate links, a shop­per who picked a size and color on Yandex.Market lands on your prod­uct page with both already selected, and sug­gests build­ing the links from the color and size, e.g. http://best.clothes.ru/product_page.asp?pid=12344&colour=14&size=44.]

Google Merchant Center takes the same view:

For vari­a­tions of the same prod­uct, we rec­om­mend you submit URLs that pre-select the vari­ant which the user clicked on in Google Shopping.

On the other hand, it means reload­ing the page when­ever a size is picked, which is a dis­gust­ing user expe­ri­ence and eats a lot of resources.

Can you use AJAX to pick a size and swap the URL? I think not. My knowl­edge of tech­nol­ogy tells me AJAX means no page reload, and how do you change the URL with­out a reload?

Besides, you’d have to hide a mass of URLs for indi­vid­ual prod­uct com­bi­na­tions from search engines.

Can anyone sug­gest the best solution?