You have two problems here:
- the spotlight is probably tungsten and probably not a photoflood, which means it will be really really orange, and
- the flash colour is different from the spot colour.
You have a few options. First of all, you can put a CTO ("colour temp orange") gel on the flash and it will convert that to a similar hue to what the spotlight produces. However, you will end up with a horrible orange image on your film. You could put a Tungsten-to-Daylight filter on your lens and end up with a vaguely-correct hue, but it's going to be ugly and you're going to lose about 2 stops of light to that filter. Getting enough light to shoot someone on stage is hard enough but shooting at the equivalent of ISO25 (Astia plus Tungsten filter) is going to be horrible-to-impossible. And you'd need huge flash power to get through both the CTO and Tungsten filters, each of which is designed to pretty much filter out everything that got through the other.
I would recommend instead that you shoot a fast negative film, e.g. Portra 400 or 800 if you want colour, otherwise TMY2 or Delta-3200 (both are good at about 1600). Keep in mind that B&W films aren't very sensitive to red/orange light from the spot, so you probably want to give about 1 stop more exposure for the spot than indicated by your meter. And you have similar problems with colour-neg film - the orange light from the spot has about 1.5 to 2 stops less blue light than red light, which means the blue layer in the film will have dropped off the toe while the red is still registering, which leads to really bad colour quality. If you're using colour-neg, give it an extra stop of exposure to be safe. Maybe also push the film a little in development, though you will have problems with high contrast already in that light.
If you have control of the stage, putting up a big softbox for your flash and using short flash-only exposures would be a good option. Probably piss the audience off though.
