The K2 (Yellow) Filter falls under the colored filters category.
As their names suggests, these filters use HOYA colored glass. They are used for color correction of different light sources when using color film, or for controlling contrast with Black & White film. Color correction filters are important as color films do not have the flexibility of the human eye to automatically adjust to different situations. Black & White films register colors as shades of grey and the rendition of each color in a scene is important, so filters can be used to control this. The color of the glass used in all these filters is carefully controlled and to reduce the possibility of color shift over a period of time, such high quality filters are coated or multicoated on both sides. This maintains the desired effect and gives a long service life.

The K2 (Yellow) filter is especially useful for clear contrast between blue sky with clouds and foreground. Provides a natural tonal rendition. Often used for subjects at intermediate distances.