I had to look up a video containing footage of Through The Eyes Of A Spiketail, as I don't recall if I saw what his vision was like or not. What I'm seeing is that he sees everything in purple. Well even close to pink. Purplish pink.
This also can be explained by the number of cones. Remember what I said before, the more cones you have, the more color you see, and the less you have, the less you can see? The same thing applies here.
What Spike has is color blindness. This condition is caused when one or more cones are missing. It doesn't require you can't see any color at all (though that does happen sometimes). For instance, you could be missing the cones required for seeing green, so anything that appears green will appear to be brownish or some other color to you.
Spike appears to be missing most of his cones except for perhaps his ultraviolet ones, or part of a cone that lets him see the pinkish purple coloration. As a result, Spike has monochrome vision (shades of one color) and is completely unable to see any other color besides pinkish purple.
He may have been born this way. If not, something happened to him to cause it to happen.
