Got it, thanks!
Looking at it in 3ds Max, I can see why its not working. Although the UV map does fit correctly, it isn’t even close to being a grid and there are huge areas of the UV map with no vertices in them.
Here’s a common sense way to think about it. To get the rainbow effect, you need lots of different colors. Each vertex is represented as a color. So, if you don’t have many vertices, you don’t have many colors.
I don’t have time to say any more right now, other than to say that your best chance of success is to start by importing a pre-existing sculptie and modifying it.
Next week, I’ll try to spend some more time with it so that I can make some more helpful recommendations.