What is the purpose of color-adjusting light sources with gels in digital photography? Are they reserved exclusively for scenarios in which one wants different colors from different lights?
I assume that if one wants to affect the overall color cast there is no advantage to doing that with lighting gels as opposed to applying it digitally in post. If that assumption is incorrect why?
Yes, their main purpose is to have different colors on different lights. However in the vast majority of cases (if not always) you simply cannot reproduce this setup in post.
The human eye is quite good at detecting natural light falloff and it will detect the things which are Photoshopped, especially if we talk about a setup with multiple lights (we include here the ambient light as a separate light source) and uneven surfaces.
Also, having the gels in-place you can tune up your scene with great accuracy in order to obtain the mood/message you want, including adjusting your subject(s).
Again, having real gels will give you the possibility to re-shoot the scene in different ways (different angles, subject positions, etc.) in order to have options from which to choose.
Generally speaking, the old rule still stands: Get as much as you can straight off camera and use post-processing for minor adjustments.
From my point of view - or how I use my gels - there are two main usage points:
Adjusting a color to get a color effect. For example make your flash-light red/green/... to get a interesting background color spot.
Adjusting flash color to the color of ambient light, so that your picture have only one color of light. If you have different light-colors in one picture it is hard to choose "the right" white balance. On Digital Photography School you can find an good post about this.
If light colors don't match (or you don't want them to match and they do) then you can't correct for this in post. Lights interact with each other and there is no good way to tell which light is contributing where in a reliable and automatic manner in post. This means that you can not adjust the color characteristics of an individual light after shooting.
If you want to impact the color of only one light (either to bring it in line with other lights or to knock it out of alignment with other lights) it must be done with gels at the time of shooting.
One main reason color gels are used on flashes is to match ambient lighting so that white-balance correction doesn't become a post-processing nightmare. When you use a flash, the color temperature of the light that comes from the flash is often different from that of the ambient lighting. And the mixed sources of lighting will both hit the same subject and mix/blend/gradient depending on position, diffusion, and intensity. Unless you're extremely skilled as a retoucher, and the light didn't mix too liberally, you might be able to get away with it in post with careful use of masks/blending. But it's easier to simply have everything the same color, and then adjust everything together, rather than adjusting for the blueish tint of an ungelled flash, only to find all your tungsten ambient light going more orange, or correcting for your tungsten ambient, only to have whatever the flash lit going even more blue.
The other reasons is to do the opposite. If you want to make the entire background bluer, then you throw on a CTO gel, light your subject with it, and when you color correct everything the light didn't hit becomes bluer.
You can also use gels to separately color a background, so long as you don't contaminate it with your other lights.
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