I can't speak to Super Mario World as I haven't printed any SNES covers yet, but I meant that I was personally having issues with my printer - i.e. low ink, my printer is crappy quality. But I've never had any major issues printing the UGC or BitBox versions of these covers on inkjet before. Sometimes there are instances where a color appears a little darker but I don't recall any where it was enough that it seemed drastically different. Now laser printers have given me all sorts of trouble printing these and other covers with very dark colors.
Before I got super busy, back around January, I went to the Fed Ex print shop hoping to test print and eventually have all my covers printed there so I didn't have to mess with it. Unfortunately they were all too dark (and their machines really wanted to resize the covers too) so I gave up and went back to my inkjet. If any covers print out really odd, let me know and I'll adjust them.