You can control something to go through the spectrum with a single control. It can then adjust only hue, not brightness or saturation. If the control went from 0 to 3V, it could work like this:

0 = red

>0 < 1 fade red to green

1 = green

> 1 < 2 fade green to blue

2 = blue

> 2 < 3 fade blue to red

3 = red again!

So for example, in the range 0 to 1V you would set the duty cycle of PWM for the red LED to 1 - v and the duty cycle for green to v. In the other two ranges, you would subtract the base of 2 or 3 and then do the same with the fade from / to colors.

For example, 1.3V: The range is green to blue. Subtract 1 to get 0.3, so the PWM signal for green is 70% and for blue is 30%.

Bob