Better cycling through the RGB colors
I think it's worth noting that the three 255 values for RGB colour space is just one way to encode the 255^3 (16,581,375) colours. These include each and every brightness level too. 

If you are trying to get a rainbow effect, you may have more success or find it easier to use a HSL or HSV colour space.

HSV uses Hue, Saturation and Value instead of RGB. FastLED has optimized it's implementation of HSV to use a 0-255 addressing for Hue (instead of 360), keeping it fast for animations run on microcontrollers:  

https://github.com/FastLED/FastLED/wiki/FastLED-HSV-Colors