Keeping greenhouse warm with lights

[Raqsharqi]

Our overnight temperatures are supposed to drop into the 40s. My 8x8 greenhouse doesn't seem to hold any heat in at all overnight, and I'm afraid the 40s will be bad for my orchids. If I only need a bit of heat, is it safe to use light bulbs to keep things a few degrees warmer? If I do that, do I have to use red or yellow bulbs so that photosensitive plants do go all wacky on me?

[Fairorchids]

How about an electric heater controlled by a thermostat, plus a fan to distribute the warm air?

[silken]

I like Fairorchids' idea better than using lights.