I would use newt's formula, but for it to look like it was blending between colors as opposed to traveling through all colors to get to the right one, you need to lerp each component like
rgb(lerp(0,255,t),lerp(0,50,t),lerp(78,34,t))
You can make a pv for the value t, and increment it every tick, or you can use the math plugin to do an autointerpolation of a value "t", from 0 to 1, and give the total time, and have it automatically increment the value t which you could put there as well