Theatrehelper posted great insight!
Kathie by ‘centre spot’ I assume you mean a ‘lime’ [follow spot]?
I personally I only use a limes  for solo/duo song pieces in musicals.
 Depending upon the wattage of the lime, you may have to fade down the general wash in order to use the lime.
This is dictated by the mood of a hard edge spot or tweaked out of focus to soften the edge up. Normally the Iris comes into play & can be used effectively to draw the punters focus. Even to dictate the time lapse in heart beats, before any applause is given at the finish of the song.
With a good spot operator there are a lot of tricks & effects that can be employed, using the Iris or Dowser.
With the witch I’d use Congo Blue’ [L180] the colour gels can be found in a gel swatch book or on these sites for Rosco & Lee;-
http://www.rosco.com/uk/filters/supergel.asp

http://www.leefilters.com/LP1.asp?PageID=21
 
However you need to decide on your general colour wash for the Cyc, ground row, boarders, Side, floats & FOH lighting.
This will give you the colour palette to mix from & add in Specials to high light. As for the emerald city & Kansas you could go with Cyc colour  of green for Oz or Amber’s/Blues for Kansis, along with the mise en scene covered by general wash.
As theatrehelper mentioned it is a case of playing about & experimenting with it.