I'll add to the pile.
The color of light is measured along two axes: the yellow/blue axis and the green/magenta axis.
The yellow/blue axis is measured in degrees Kelvin (˚K). This unit of measurement is used because Lord Kelvin determined, among other things, that as a black object (Carbon in his experiments) was heated, it exhibited a warm, red glow at high temperatures (about 2000˚K). As the object became hotter, it glowed more yellow. Even hotter and it started to exhibit a bluish tint at about 5000˚K. Even hotter and it became even more cool/blue. So, film & camera makers decided years ago to balance films to certain "color temperatures". This would allow the film to record neutral colors when used in the proper lighting environment (e.g., indoors with tungsten lights, or outside under the sun).
This yellow/blue axis is sometimes referred to as a "warm/cool" axis, and judicial use of CTO and CTB gels can give the illusion of an indoor or outdoor scene, or can help in setting a tone for a certain time of day.
The green/magenta axis was incorporated by color scientists (and film & camera makers) to account for the color shift of light at a specific color temperature. Color scientists determined this green/magenta axis to be perpendicular to the yellow/blue axis. The green/magenta color shift is independent of any color temperature. It's usually a result of other causes such as weak or strong phosphors in a lamp, or a light manufacturers bulb coating, or debris in the atmosphere (e.g., smog), or a lens manufacturer's glass. The green/magenta axis unit of measurement is a "mired".
To get neutral color, a good color meter and accurate use of CTB and CTO gels can be used to great advantage when using film in achieving neutral color. When shooting digital, a neutral white board (one without optical brighteners) can be used to set the white balance in-camera. (However, additional color balancing or grading is almost always done in post).