The colder the lighting, the higher it is on the scale and the higher its value in Kelvin (which can be confusing, as when you usually measure the temperature of something the rule is: the higher it is, the hotter it is). The white in the middle of the scale represents a neutral colour temperature.
The white balance on a camera is what controls the colour temperature of the shot. White balance is the process of manipulating the colours from a light source, so that only certain colours are seen. The camera needs to be told what is white in the frame, so that it can calculate the correct colour of everything else. Most cameras have presets, such as daylight and fluorescent.
These presets should really only be used as a frame of reference, as they aren't 100% accurate for every situation. For example, using the Daylight preset on a very sunny day will produce different results than when you use it on a less sunny day. Entering your own values for the white balance is the best way for you to produce an accurate white balance. An easy way of setting your own white balance is to hold a piece of white paper in front of the camera and use the eyedropper tool to tell the camera that that is what white looks like. However, if you do this and then decide to go back to a preset, the preset will produce an interesting effect, because you've changed the camera's value for white. Therefore, if when you used the daylight preset before setting your own white balance and it was slightly orange, after setting your own it may be far more blue and cold.
Without correctly white balancing a camera you may end up with an unwanted effect, such as a very orange shot during a "sad" sequence. You want to be able to manipulate the colours of the lighting so that you can control all light sources (such as the sun).
If, after setting up your lights, you are unable to produce the desired effect with the white balance on the camera, you can use gels. Gels are sheets of colour that are used to manipulate the colour temperature of the light. If you place a CTB (colour temperature blue) gel in front of a Tungsten (an orange/yellow light), it cools down the colour temperature without having to mess with the white balance settings on the camera. There are also CTO (colour temperature orange) gels and neutral density gels (to bring down the general composition of light if it's too harsh or bright).
The use of gels, specific lights and the manipulation of the white balance can create a day-for-night effect. Basically, if you're wanting to film during the night, you're going to need to use multiple big lights (often Tungstens) so that the audience can see what's going on during the scene. However, you still want it to look natural - you want the light to look like moonlight. We perceive moonlight as blue, even though it's not, so you can use gels to combat the orange light (roughly 3800K) of the tungstens (or use large fluorescent lights if possible). For a day-to-night effect you also probably want to alter how wide the barn doors are open on each of your lights to create a realistic light effect. In the film 'The Antichrist', there is a scene that uses the day-to-night effect really well. The use of cool, fluorescent lights creates a moonlight effect on the water of the river and the subject in the forest. It's lit enough for the audience to understand and it's dark enough for it to be a realistic night scene.
On the other hand, day-to-night conversions don't always look the way you want them to. Depending on when and where you shoot your scene, you may end up with a poor end result. If you are actually shooting during the day somewhere quite bright, adding a blue filter to your lights will not make day look like night.
It may seem fairly convincing during the shoot, but once you get to post production you're going to realise that the audience is going to know that you shot it during the day. Why? Because the night isn't just blue. You want the shot to have a balance between a cold colour temperature and darkness. You only want the subject to be lit up during a night scene, which is what you'd use the blue gel/ change the white balance for.
Sometimes people change the white balance so that it's purposely wrong. This can create an abstract/ artistic effect.
For example, we know that the lighting in this shot is artificial, because daylight (which we can see reflected on the table) isn't blue like this. Despite this, the shot still looks right, but in a slightly more edgy way. This can be used to influence the mood of an audience or create a cinematic effect.
