Which hair colors are easier to switch between?

Well, blue tends to fade out to a greenish hue, depending also on the dye and your hair base.

Pink on the other hand fades to peach.

Now, pink cancels green out, and blue cancels peach out, so either way the color will turn grayish/dusty after the transition, no matter which way you go.

I believe both these face out pretty fast, so it really shouldn't be an issue no matter which way you go!

The reason that most blues fade to green is that the hair usually has some yellow in it.  Blue + yellow = green.
And the pink dyes fading to peach?  Same thing.  Pink + yellow = peach.
(Obviously this varies depending on base colours of hair and the dyes as well)

You could use a pastel purple as a transition point?  Just mix the pink & blue dyes.