Gelling to ambient colors

[mpistone]

Hi everybody
I'm still learning to use my speedlight, and so far I've found that if I use a magenta gel on my subject with window light coming in against the background wall I can do two things:
1) Leaving my WB for daylight I get a magenta subject and white background.
2) Setting custom-WB with the magneta flash (by shooting the inside of my umbrella and using that image for custom), I get accurate skin tones on the subject but a green cast on the background and anything else that's mainly lit by ambient).

My question: besides trial-and-error, how can I tell what my colors will become using the second method? Suppose I'm outside and I want to give the world a purple color but keep my subject fairly 'neutral', I would guess a green gel would do that (just like a purple gel did the opposite), but is there a method to use? Something using a color wheel maybe?

I'll post some examples at some point but I'd appreciate any thoughts in the meantime also!

[Wilt]

Why are you trying to use a green gel or a magenta gel on a daylight-balanced light source?

One usage would be if you had fluorescent ambient lighting, then you would use a light green gel to make your flash similarly greenish, so that a single color balance adjustment in post processing would correct all sources to daylight.

A different usage of gel would be if you had fluorescent source, and a large magenta (about 30M) filter that is put over the fluorescent, so that it is closer to daylight balance, then the flash and the fluorescent both are naturally correct (daylight).

[Milamu'g]

Why are you trying to use a green gel or a magenta gel on a daylight-balanced light source?
...

To affect the color of the ambient like the OP stated. If you want the ambient to show up as purple you can either put a purple gel over the ambient, not handy when the ambient is the sun ;D or can adversely gel the flash, set WB according so you retain correct skin colors. The question was is there a quick way to tell what the ambient is gonna do when gelling the flash, which would depend on the color temperature of the ambient, the gel on the flash and WB setting.
As for the OP's question;
I'm sure there are more scientific approaches but I would say trail and error and then experience.

[Wilt]

To affect the color of the ambient like the OP stated. If you want the ambient to show up as purple you can either put a purple gel over the ambient, not handy when the ambient is the sun ;D or can adversely gel the flash, set WB according so you retain correct skin colors. The question was is there a quick way to tell what the ambient is gonna do when gelling the flash, which would depend on the color temperature of the ambient, the gel on the flash and WB setting.
As for the OP's question;
I'm sure there are more scientific approaches but I would say trail and error and then experience.

It is obvious that he wants the change the light...I was inquiring about under what situation does he feel the need to do so.

[Hermes]

You should at least commit to memory that Red and Cyan are opposites, Green and Magenta are opposites, Blue and Yellow are opposites. From there you should be able to work out what gel you need to use to shift the ambient colour.

[mpistone]

Wilt, as I said the reason I want to do this is to give the ambient light a difference color when I don't have control over the ambient source.

Thanks Milamu'g and Hermes, that's what I was looking for.

[GerBee]

Wilt, as I said the reason I want to do this is to give the ambient light a difference color when I don't have control over the ambient source. Thanks Milamu'g and Hermes, that's what I was looking for.

With practice you'll find it is variable and whilst your camera may have a wide K setting, the filters will typically be in three strengths but can be stacked.

In the 70s it was all the rage to use vivid red, blue, yellow on the lens and its opposite on the flash ~ mostly overused by fashion photographers as it was quite a spectacle ~ then interest rather fizzled out.

The technique is still alive and use daily, it's a favourite trick on the Strobist forums. :)

[Wilt]

Wilt, as I said the reason I want to do this is to give the ambient light a difference color when I don't have control over the ambient source.

Thanks Milamu'g and Hermes, that's what I was looking for.

That is still a general description. Selection of gels is quite specific to the situation at hand, so no general reply provides any useful answer at all. That doesn't provide specific solution for a specific problem at hand. For example,

Color balance alteration is one application
Color rendition (applying a hue) is a different application

In my original reply I gave two specific uses, relative to the first application. For that application, the precise type of light must be considered, as gelling a home bulb is very different than gelling a photoflood bulb, if you wanted to achieve daylight balance.