Either:
a) projected from high powered projector
b) projected from Source4 (or other
ERS)
c)several ellipsodials from somewhere (probably the pipe you use for your
cyc lights). Half red, half n/c, half blue. Just use the shutters to draw your lines in, and use however many fixtures you need.
d) use normal cyc lights
e) paint a drop
f) paint your cyc
IMHO:
Choice A would be hard because I presume your going to have light and actors on stage at this time, so your not going to have a powerful enough projector to make it work.
Choice B has the same issues as choice a
Choice C IMO would be the best idea. Get a bunch of Source4 or similar ERS and
gel them the necessary colors. At the points that colors meet, just use the shutter to get a nice hard edge. When choosing the position of the fixtures, remember that if you put the fixtures right on top of the cyc your not going to have a sharp line going down, and the further away you put the fixtures, the
dimmer the light and the more likely it will be that someone will walk into the beam.
Choice D would kind of work, but you wouldn't get the distinct line that you want.
Choice E is the best option
Choice F, well, if your going to consider choice F, send me your cyc, I could use one.
Not to ask a stupid question, but why does Once On This Island need this?
ON A COMPLETE SIDE NOTE, AND NOT TO HIJACK THE THREAD:
Les Miz on broadway (not whats on now, the original) had a great effect at the end of Act 1 when they hazed up the entire stage fairly well, and shot the french flag into the air. As far as I can tell, they used their entire toplight
PAR can wash (or whatever it was) and had them in the colors of the french flag. However, the lower section of the stage was lit by front and low sides so that there was the french flag hanging in the air, and a normally lit company marching under it. Gorgeous effect.