Pesonally, I would never think about doing any color-critical work using a MacBook. The MBP, perhaps. There's been an on-again-off-again dispute over whether MB screens can even display "millions" of colors. I seem to recall them only being able to do "thousands" in the control panel.
In any case, I've never been pleased with the color fidelity I've seen on MBs. I've never seen them run an external screen, but I can't imagine it would be any better. If it looks dodgy on the MB's screen, I can't imagine things being better on an external screen.