I've raved about this for ages. It's the nature of paper when it's a pure white variety. Over time, with environmental factors accelerating the issue, the chemical compound of the paper breaks down and looses it's vibrant white appearance. It doesn't necessarily turn yellow, it just becomes less and less vivid as it breaks down and becomes less stable. Compared to the white plastic of the case, it will look like a different color eventually.
Has happened not only to my Wii artwork, also my Sega Saturn artwork, Dreamcast artwork, the white banners on Xbox 360 games, PS5 games, etc. Anything vivid white, will eventually become dingier white over time. A lot of my PS5 games are already doing this. I live in an environment where air humidity levels tend to fluctuate from dry to normal-borderline damp, and that's a nightmare scenario for thin little slips of high gloss paper. Hell, all of my handheld console screens turn yellow over time too, because of these dry to damp shifts. It's a common problem in Japan due to their climate conditions as well.
You have two options. Just accept this shit will turn dingy and look bad, because you know it's something that happens outside of your control. Or, use a scanner to do high-res scans of all of your artwork (avoid the low-res crap found on most websites, because it'll look like a cheap counterfeit), photo edit to tweak the dingy tint back to white, then reprint all of your artwork. It'll be good for a while, but it'll just happen again eventually. Still, you can do a refresh as many times as you'd like ones you've got all your stuff archived...
A third option, buy some cream colored DVD cases?
