I tried and failed to explain this to people once. It’s not that a magical never before seen color exists outside our visible spectrum, its more like their red and violet ends would be shifted further along the spectrum and everything inbetween would be more differentiable. A great example is migratory birds can tell North from South by the color of the sky, because the electromagnetic field around the earth has a color gradient. It just all looks the same to us because by comparison we’re colorblind to that part of the spectrum.