Arduino RGB spectrum problem
(1) None of your Inc / Dec functions use the argument that is passed to them. Not sure if that was intentional, but it's noteworthy.

(2) Your Dec functions have a nonsensical termination condition in their loops. Instead of &lt;=255 surely these should have &gt;=0?

(3) The way you wrote your loop it's basically going to do the following sequence (after you fix (2) that is):

#000000 // BLACK
#010000
...
#FF0000 // RED
#FF0100
...
#FFFF00 // YELLOW
#FFFF01
...
#FFFFFF // WHITE
#FEFFFF
...
#00FFFF // CYAN
#00FEFF
...
#0000FF // BLUE
#0000FE
...
#000000 // BLACK


Not sure that's what you intended, but there you have it.

(4) Don't say z += 1 and z -= 1 in your for loops, say z++ and z--. People who read code will raise their eyebrows at the prior.