Will rapidly changing cycling an LED through different colors reduce lifespan?
It would likely decrease reliability a little bit. Temperature is a much larger factor in LED reliability. LEDs are much more complicated than they appear at first glance. There are many factors with the pulsing that would need to be known. (e.g. Pulse duration, current amplitude, current ripple, frequency,and duty cycle).

But compared to temperature, pulse is negligible when the LED is driven below Imax. If the LED is not too hot to touch (50º C), it is running way below Imax. I max would drive temperature to 125º C or more without significant thermal management. 

I am currently writing a paper titled: Understanding LEDs.