Tween the color difference in the 3 components with the actual value of [0..1]
for example
c1: 200,20,50
c2: 100,200,160
=> dif: -100,180,110
act color = c1 + tween * dif
for example
c1: 200,20,50
c2: 100,200,160
=> dif: -100,180,110
act color = c1 + tween * dif