Shooting outside the studio comes with a lot more variables you will have to contend with when lighting your shot. A common situation is having multiple light sources that don’t exactly match in color temperature. This could be cool daylight from a window and warm tungsten practicals inside or it could be that two lights you are using have a slightly different native colour temp and you want them to more closely match.
What is mixed lighting?
Mixed Lighting is a lighting situation where more than one type of light is present. It can be intentional, or a problem that needs to be solved either on location or when post-processing. An example of intentional use of mixed lighting is the use of strobe and continuous lighting together in a studio setting for special effects. This combines a long enough exposure to show motion with the frozen image provided by flash. An example of undesirable mixed lighting is the use of tungsten light in a room with daylight coming through a window. No matter what white balance you choose on your camera because the two light sources are a different color your resulting image will have parts that are either blue cool or orange-red warm.
Photographically speaking, mixed lighting refers largely to the color of two different light sources, more so than any other aspect of differing sources of light such as flash versus natural light, or bright versus dim, etc. even though technically, any two different light sources could be considered "mixed".
Daytime ambient sunlight, for example, is roughly 5500 Kelvin white balance temperature. Indoor lighting that is roughly similar to the traditional Tungsten color temperature, on the other hand, will be in the vicinity of 3000 Kelvin white balance.
If you want to learn how to work with mixed color temperature lighting, here are some expert quick set of tips and techniques that will help.
When shooting in mixed light, if you match your white balance to one of the light sources, that light source will appear neutral in colour while the other light sources show their true (relative) colour. For example, shooting an indoor + outdoor scene with daylight white balance set would render the outdoors "normal", while the indoors would be rendered with whatever warm colour is coming from the lamps indoors. Oppositely, if you set your white balance to an indoor setting, (incandescent, tungsten) then that light would appear more neutral, while the outdoor light would appear extremely blue.
If the difference between two light sources is great enough, (dusk light is far bluer than normal daylight, for example), therefore splitting the difference between the two light colours would result in both very warm colours indoors and very cool colours outdoors.
When processing a raw photo that is captured in mixed lighting, the same constraints apply when setting your overall white balance. If you match one light source, the other will exhibit its relative colour. To fully correct this and match light colours, localized brushes and/or selective colour adjustments would need to be used, to cool down or warm up any undesirable colours.
Alternatively, if a special effect is desired, pushing the white balance further in one direction can result in a very dramatic colourization.
Probably the most common issue is having windows and doors in the background. Usually, this goes beyond the dynamic range of modern digital cinema cameras and the best way to solve this issue is to use another light on your subject to balance the exposure. Now, if you use a tungsten light you’ll notice that it looks very warm compared to the natural daylight. It’s very noticeable and can make things look unnatural.
To correct changes where you just want to do a simple conversion from tungsten (~3200K) to daylight (~5600K) you can use a gel. A CTB (colour temperature blue) gel is designed to do just that. The only thing to consider with gels is that you will lose some brightness with gels as they absorb light to make the color shift happen.
A modern alternative is to pick up a naturally daylight-balanced fixture for these scenarios. Or, better yet, get an LED with an adjustable colour temperature. Variable colour lights will be able to be balanced in most situations as you can tune them to the exact shoot.
Honestly, if you are looking to purchase lights today you should just go with some LEDs. I’m not so convinced you need anything more than daylight-balanced LEDs to start (brighter and more colour accurate than variable colour temperature LEDs), but the new full colour LEDs offer a much wider range of creativity.