Which of the following colors will result in a shortened processing time when using a lightener on grey hair?

When using a lightener on grey hair, orange is the color that results in a shortened processing time. This is because grey hair is often resistant to color, and using orange as a tonal reference helps to effectively neutralize any unwanted undertones that may arise during the lightening process.

In color theory, orange is situated directly opposite blue on the color wheel. When lightener is applied, it lifts the natural pigment from the hair. Grey hair, being devoid of pigment, can sometimes take longer to lighten fully if a cooler color is used. Orange acts as a warm base that facilitates a quicker lightening effect, thereby reducing the overall processing time compared to other colors.

Yellow, red, and blue do not provide the same efficiency in processing time. Yellow, while a lightened shade itself, may not provide enough of a contrast to effectively activate the lightening process on grey hair. Red can contribute warmth, but it does not neutralize effectively and may create unwanted tones. Blue is a cooler tone that can counteract orange or red hues but doesn't facilitate faster lightening on its own. Thus, orange is optimal for achieving quicker results on grey hair when lightening is applied.