With double the strength of the KR3, the B&W 77mm KR6 Warming Color Conversion Filter leads to neutral color rendition in strong haze or in mountains above 2500m (8200') when transparencies are not to show "blue mountains" (which can also be attractive depending on the individual case).
Another field of application would be interior photographs on daylight reversal (slide) film when only blue light from the sky and no direct sunlight is coming through the windows - and no fill-in flash is being used. In combination with tungsten reversal film, the KR6 filter imparts the warmth of normal incandescent bulbs to the light emitted by halogen bulbs which would appear neutral white without a filter.
Note:
Whereas the 81 Series of filters creates a feeling of warmth, the 82 Series creates a feeling of coolness
81 Series Conversion Factors
81 decreases color temperature from 3300 - 3200 Kelvin
81A decreases color temperature from 3400 - 3200 Kelvin
81B decreases color temperature from 3500 - 3200 Kelvin
81C decreases color temperature from 3600 - 3200 Kelvin
81D decreases color temperature from 3700 - 3200 Kelvin
81EF decreases color temperature from 3900 - 3200 Kelvin