The viewport seems to do weird stuff when your vertex colors are not in the [0,1] range. I noticed that negative numbers or larger positive numbers made the viewport appear to use some sort of modulo effect on the displayed color. I assume its running afoul of something like this:
color = (int)(uvw[0].x * 255.f) & 0xFF;
The summary of my story is that Max is weird and I didn’t do it.