It might seem like a subtle improvement to you but I really think it actually achieves the goal of making it so that you can kind of get the structure of the code just from glancing at the colors.

Structure, from colors. I don’t know, I have never tried to infer structure from colors in the code.

This is what prism.el provides: color reflects logical depth, which signifies the primary aspect of structure. IMHO it is generally more useful than making function names stand out (their position makes them stand out anyway), variable declarations a certain color (though helpful, I don’t need to see that constantly), etc. A well-written piece of code communicates a lot by its shape, which logical colorization helps to reveal.