Vary step interval, not step size. exponential timing.

Changed the code to always step by increments of one, but step some colours more frequent than others.

I used prime numbers so you get a pretty long cycle.

I added a table, mapping linear values to exponential ones, to match the human eye.

I'm still contemplating if I should correct the brightness of the different colours. Right now, there is a lot of blue, and not so much green.