First of all, you need to understand what the Kelvin temperature
scale is.
The Kelvin scale is a scale for measuring temperature from absolute
zero, the temperature that scientists believe is the coldest possible
because absolutely no heat energy exists at that temperature.
The steps from degree to degree in the Kelvin scale are the same
size as the steps in Celsius, but 0' Kelvin is absolute zero.
Imagine now that you were to take a lump of carbon called a black
body and start heating it, all the while measuring its temperature
in degrees Kelvin. As the black body gets hotter and hotter, it
will begin to glow red. Once it starts glowing, heating it more
will make it change colors. When it reaches 3200K, it will glow
orange. When it reaches 5600K, it will glow blue.
At some point somebody decided that using those temperatures
would be a good way to describe those colors in other sources
of light, and that method of naming the colors became color temperature.
Thus, when someone refers to 3200K light, it means that the light
is the same orange color that a black body would glow when heated
to 3200 degrees Kelvin. Now that we have a scale to describe the
light, we can measure color.
That's the source of those numbers on your camera that correspond
to the filters. A 3200K filter is for light that is closer to
orange. A 5600K or 6400K filter is for light that is closer to
blue. But your eyes automatically adjust to see different light
sources as "white," so how do you know when to use which filter?
First, most tungsten light is between 2700K and 3400K. The light
bulbs you screw into sockets in your house are tungsten. The quartz
bulbs you put in a Lowel light kit are tungsten. For this kind
of light, you'll want to use your 3200K filter on your camera.
Direct sunlight is usually around 5600K. Shaded areas outdoors
during daylight can have higher color temperatures, up to 10,000K,
making them appear more blue. For these lighting conditions, you'll
want one of your daylight filters, which will probably be 5600K
on a single wheel system.
But what about these other markings on the wheel, the ND settings?
ND stands for neutral density. Neutral density is a grey filter
that should not affect the color of light passing through it in
any way. You can think of it as sunglasses for your camera.
If you are outside in direct sunlight, the light will probably
be too bright for your regular 5600K filter. You need a way to
knock down the light so that the camera can handle it. That's
where ND comes in. Cameras usually have the ND expressed in fractions,
such as 1/4, 1/16, etc. What that fraction means is that the ND
filter is dark enough that it only lets in that fraction of the
light hitting the lens: a 1/4 ND filter means that only 1/4 of
the light coming into the lens is being allowed into the camera.
Most single wheel cameras have the ND filters combined with the
daylight filters; you'll usually have one 5600K filter with no
ND and two filters with different levels of ND on them. One of
them will be intended for moderate daylight, while the other will
be better for bright, direct sunshine.
It's useful to understand where all those numbers come from,
so if anything doesn't make sense, please ask questions.
Now let's look at practical application. 
First,
forget about any advice on which filter to use indoors or outdoors.
Don't think about whether you're inside or outside, but rather
what color the light is wherever you are.
With that in mind, if you're shooting under tungsten light, you'll
usually want to white balance on your 3200K filter. You can have
tungsten light inside buildings, or outside at night.
If you're outside under daylight, you'll usually want your 5600K
filter. But what about inside a building with a big window? If
the room is lit mostly with daylight from the window, you still
want your 5600K filter.
Flourescents can be a bit tricky, because they come in different
colors. In offices, where we usually encounter them, they are
either 3200K or 4300K. In either case, you can use your 3200K
filter. However, some hospitals have 5600K flourescents in certain
wards that require your 5600K filter. If you get an error message
when white balancing on your 3200K filter in a hospital, that
may be the problem.
Also problematic are gym, hangar and stadium lights. Sometimes
these, too are daylight, and will require your 5600K filter.
On the opposite extreme are sodium vapor lamps, those horrible
orange/red streetlamps. These will often be as low as 2300K, and
will sometimes give you error messages telling you the color temperature
is too low. Just balance on your 3200K filter and hope for the
best.
The worst situation is mixed light sources. You might be in a
room that is lit on one side by daylight from the window, but
lit on the other side by tungsten. Which filter do you choose?
You have to white balance on the correct filter for whatever you're
shooting. Thus, if you shoot something by the window, you need
to be on your 5600K filter; if you move over by the door where
the tungsten light is the strongest, you'll have to switch back
to your 3200K filter.
If this sounds complicated, don't worry; you'll learn it by doing
it. It's easier if you learn to override your brain's tendency
to process everything as white and learn to really see the colors.
If you have access to any tungsten lights, a good start is to
take one outside during daylight, point it at the ground and pay
attention to the difference in color. Then set it up under some
flourescents inside and do the same thing.
Another good exercise is to try to guess what color the light
is before you white balance. Look at a room and say, "I bet these
flourescents are 4300K." When you white balance, most cameras
will tell you in the viewfinder what color the light is. Guess
first, then see if you're right.
If I haven't lost you yet, there's one more thing I should clarify.
We often refer to colors of light in terms of "warm" and "cool."
The problem is that we refer to them backward from the way the
color temperature works. We say that blue light is "cool," while
orange light is "warm." But blue light has a higher color temperature
than warm light. It seems ass backward and counterintuitive, but
it has to do with the way heat is transmitted and works out okay
if you just remember that those two ways of referring to color
work in reverse of each other.
And one other thing: I haven't mentioned the numbers of the filters
themselves (i.e. 1,2,3,4 or A,B,C,D). I'm leaving it up to you
to take what you've learned here and apply it to your specific
camera, figuring out which filter corresponds to which type of
light depending on color temperature.
That's a lot to digest, so please ask questions.
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