shenske: I think that looks great, but I don't think the spine color shouldn't bleed into the back cover. You can leave that black as it was before.
Pleaaase no pink.... 
The reason Mario USA spine is pink is because the actual cart is. Google Mario USA Famicom and you'll see the pink. Even the original box was pink!
Jigsaw:
I hear what you're saying and I agree with you with some of what you said Jigsaw, but the whole charm of Famicom games as any Famicom collector will tell you is that the games
aren't uniform. As I said, nearly every game cartridge is a different color from the last. Think SNES Doom or a few N64 games that were different colors, but apply that on a much grander scale.
Jigsaw did bring up a good point about the odd time a game cartridge will clash with a logo, but I think this is easily fixed by simply adding an additional outline of a natural color such as white to separate the logo from the rest of the spine so that it does stand out. In extreme cases (rare gold colored carts or ones with multiple colors) I agree that the spine should be changed to something that would work better, but those are rather extreme examples. Although yes, Famicom games can be any color, the bulk fall into the five basic colors: red, green, blue, black and white. Since none of those clash with the template, I don't see it being much of an issue. The only time it wouldn't really work would be in the case of pirate multicarts, but I don't see us ever hosting covers for those so again, non-issue.