So, assuming that you are working rounds 12 sc around, what I do is
Round n+1: working with color 1, sc 12
Round n+2: working with color 1, make first half of sc, working with color 2, make second half of same sc, sl st 1, sc 10
Round n+3: working with color 2, sc 12
(Although, really, if you're working in a spiral, doing the color change at the start or the end of the round doesn't really make the transition appear any more or less smooth. It just shifts where the transition occurs to the left or right.)
Color change location
So, assuming that you are working rounds 12 sc around, what I do is
Round n+1: working with color 1, sc 12
Round n+2: working with color 1, make first half of sc, working with color 2, make second half of same sc, sl st 1, sc 10
Round n+3: working with color 2, sc 12
(Although, really, if you're working in a spiral, doing the color change at the start or the end of the round doesn't really make the transition appear any more or less smooth. It just shifts where the transition occurs to the left or right.)