The same agents added to tread rubber that make it black also improve the aging properties. So a black walled tire will age better than a true gum wall (para rubber) which is worst, and a gum colored wall which will be in between. Most lower end tires are molded in a single pass, so the tread and walls will be of the same material, while better tires usually have tread and wall rubber of different materials, though I have no idea if they age the same or not, though it's probably close in any case.

In any case, it barely matters. The only purpose of the rubber on the walls it to protect the body plies from the elements and the occasional object that gets flipped up as you roll over it. As such, even ugly, dried out para rubber walls still do the job relatively well.

Whatever you end up buying, try to arrange long term unused storage, with the weight off the tires, so the walls don't get folded on themselves at the bottom. (or pump them once a month or so).