I heard about this, but I don't know if it's actually true: but are the colors/graphics actually kind of worse on a 3DS XL? I mean, it's what I've heard anyway, and I haven't really tried the XL firsthand, so I'm not sure about this. I was never really a fan of the XL series in general, as I got pretty bad first impressions from them(I will admit, I am somewhat quick to judge, sadly). They seemed pretty pointless, though that might be because my hands aren't really big or anything like that, so they fit snuggly on the regular 3DS.
The colors are the same. The games might look a little bit worse since they're being stretched out while being run at the same resolution, but I have yet to see a game that looks bad because of that.
I doubt the games would look any different. They just made the pixels bigger, they didn't change the resolution... right? I mean, comparing the DS to the DSi was always just fine. (I never had a DS XL so I don't know if that suffered any weird resolution changes but I don't think it did.)