As I was going through a bunch of images I hadn't seen in a while for that strange slow shutter post, I came across some in the same bunch that I shot specifcally for illustrating one of the Lighting Field Guides. It struck me as a perfect illustration for a topic that I drop and aside here and there quite often. Perfect opportunity to share just a couple of those thoughts again along with this example that visually demonstrates one of those things.
I have mentioned quite a few times that I have a little bit of a hangup about the variable temperature of little lights - especially speedlights. They are just way way too cool compared to real sunlight for my taste. I don't like it and I especially don't like that they change depending on power output. I also pretty much despise softer shadows that are warmer than harder brighter highlights. It's not an overall white balance thing it's a relationship thing that just looks all wrong to me.
I probably used an image similar to this in the Simulating Sunlight Guide to explain a few things not related to this color temp thing. May look a little different than this one due to the fact that I processed this one in Aperture 3 with a whole lot more finesse than shoot it, import it, done as I did with the guides but it's pretty much the same deal no matter how you slice it. Here is a clear reference point of just how cool an un-gelled speedlight (SB-800) is at around 1/8 - 1/4 power vs even mid-day winter sun. Running the risk of point out the obvious the highlights on the right wall are the real deal sun. The highlights on the left wall are SB-800.
Assuming you are looking at the image on a monitor where you can actually see the difference in color temperature also take note of the secondary color difference between the two. One is more magenta(speedlight) and the other is more green(sun) relatively speaking. Another way to say more green when you are talking warmer is more yellow.
This is specifically why I happen to like CTS gels in general as compared to the more "normal" CTO series gels. Most likely that's because I prefer light that feels like early light which to me looks a bit more yellow. Later light seems more orange to me. Here is something that I want to be very very specific about. None of this matters a hoot if everything is lit with the same exact color temperature as there is no difference between shadow or highlight in terms of relative color. It makes a huge difference with mixed sources. If you put CTO and CTS gels next to each other most people would probably not even see the difference.
It's when you are mixing light sources and neutralize to one that you see the difference. Hence if shadows and highlights are lit via the same color temp doesn't matter which one you use. It's a nit in terms of overall color correction. However when you mix strobe with say ambient the difference between CTS gels and CTO gels assuming that ambient is balanced the exact same between two photographs will be apparent.
About the only time I do not gel speedlights (as well as big lights to various degrees) is for demonstration images and such. If I am trying to make those flashes look natural I always gel them to some degree especially speelights they are typically at least 1000 points too cool in most circumstances. I never ever never shoot speedlights without gels in mixed lighting if I am shooting for real.
Hope this clears up the whole "color temp inversion" personal hang-up thing I drop here and there.