Q&A: Speedlight Color Shifts
The short answer is, it doesn't really vary significantly due to power level differences. But it does vary for other reasons.
So first, the power level thing.
Color temperature variances do not seem to plague speedlights as much as they do some bigger flashes. Whatever the reason, that's one thing small-flash users don't have to sweat.
But there are cases where you can get color temperature differences. And one of those instances is actually helpful. To me, at least.
The first instance is when you are cutting it close on sync speed and working at high power. Unfortunately, this is the exact combo that comes into play when you are trying to overpower sun.
Reason is, flashes are designed to give consistent color temperature over the full span of the pulse. But this doesn't mean that to color is consistent throughout the pulse. Just that the aggregate amount of light throughout will give you something (hopefully) very close to daylight.
A pulse spikes very quickly and then trails off in a "tail," the shape of which is determined by the design of the flash and the power setting at which it is being used.
If you are shooting at max power and max sync, you may well be cutting off a meaningful amount of the energy at the tail end of the pulse. And this can cause color shifts.
Not to worry -- generally they are nothing you would notice much and are easily fixable in post.
But there is one circumstance that results in a consistent (and to me, pleasing) color shift in speedlights. As flashes age, they tend to warm up. I have old Nikon flashes that essentially have the equivalent of an 81A filter (or, say, a Rosco 08) built into them due to age.
The color shift comes from the front fresnel lens on the flash. Not sure if it is purely an aging thing, or if it has to do which how much energy the plastic has absorbed over time given the large amounts of light it transmits. But it is definitely warmer.
Whatever the case, I like it. I tend to shoot people so for me an old, warmer flash is a good thing. If I were a catalog photographer, where absolute color accuracy was very important, I probably would not be very happy about it.
But my key light tends to keep a 1/4 CTO gel on it anyway, so I left anal-retentive color accuracy at the altar a long time ago.
If any of you brainiacs know exactly why a flash warms up with age (time, transmitted energy or a combo) hit us in the comments. I'd love to know