I've worked in photography and audio/visual, video & multimedia production since the early 1980s and have indepth experience to help photographers and video film makers in getting the best from their gear to create stunning images.
Basically, when you shoot a city sunset skyline there will be a whole load of horrible green fluorescent lights in the buildings - by setting your camera white balance to FL or fluorescent you dial in around 30 CC (Colour Correction values) of magenta which goes some way to neutralising the green of the fluorescent lights and adding magenta to the sky thus enhancing the sunset colour.
When setting any white balance in camera or setting a custom white balance, do remember that these settings will only effect your jpeg or preview file and not your RAW file. This technique demands that you shoot RAW - if you're not shooting RAW then now is a good time to start!
By setting a custom white balance in camera, you will be able to see a preview image of your filtration, which may then be tweeked in post-production. If you do not set a custom white balance, your image will take on the colour of your main or key light source. For these images I shot a white balance reference and then set my WB in ACR (Adobe Camera Raw) along with a few other tweeks.
White balance card on left as shot; on right corrected in ACR. Notice the sky colour and flesh tone reference.
You can take this technique a stage further to create a coloured background to a flash lit shot by using a coloured gel on your Speedlites, hot shoe flash or other flash and then white balancing using a white card either in camera or in post-production. By shooting a white card in the light with which you will illuminate your subject, you are creating a point where your image processing software will add colour to neutralise the colour cast on the white card.
If you look at a specific colour on the colour wheel, in this case green the colour at 180 degrees or directly opposite is magenta which neutralises green. By putting magenta into the image, any area of the image not directly lit by the green gelled flash will be rendered magenta.
This the scene as it appeared to the naked eye and my camera - boring.
With addition of a little light and knowhow, the ambient light becomes a whole lot more interesting.
In these examples I have used two clips/cuts of ToughPlus Green from the Rosco Strobist collection, as one (to me) seems a bit pale or insipid. Had I remembered to take along my Rosco CalColor gels then I would have been able to fine tune the background colour more precisely.
Lighting design for the above lit images
Key to Lighting Design:
Distant sky and pier.
Abi, trying not to look too bored;-)
California Sunbounce Micro Mini, supported by Laurie in VAL (Voice Activated Lightstand) mode just out of shot, approx. 8 feet/2.6 metres.
Rosco Spun Silver, white side clamped to Micro Mini frame. This gives reflectance somewhere between white and silver. Spun Silver is dual sided; textured silver one side, silver/white the other. Perfect when you need neutral reflection that's not as bright as silver or flat as white.
Canon 580 EX II, Gel Clip and 2 x clips/cuts Rosco ToughPlus Green. Manual, 1/4 power; remember the light is reduced as it travels through 2 layers of gel and bounces from the Spun Silver! Radio trigger.
Canon EOS 5D MK II, EF 100 mm f2.8 macro. Me laying in wet shingle.
Colour correction gels such as CTO (Colour Temperature Orange) and ToughPlus Green have been formulated with specific spectral sensitivities to correct particular light sources such as tungsten (Canon users) or incandescent (Nikon users). ToughPlus Green has been formulated to meet the particular spectrum emitted by most fluorescent tubes and the FL WB setting of most DSLR cameras.
There is another route to creating these effects, Rosco CalColor; The CalColor gels are pure narrow cut primary and secondary colours in 1/2, 1, 2 and 3 stop increments matched to the spectral sensitivity of colour film, digital still and video camera sensors. These filters can be combined to produce a wide variety of working filter colours.