Final answer:
For maximum coverage on 80% gray hair, a permanent dye is recommended as temporary dyes do not adequately cover gray hair. Semi-permanent colors can blend some grays but won't be entirely effective on predominantly gray hair. Permanent color offers consistent, durable coverage.
Step-by-step explanation:
If the client has 80% gray hair, for maximum coverage, it is best not to choose a temporary hair color option. Instead, a permanent hair dye would be the most effective choice. Temporary colors are designed to wash out after a few shampoos and generally do not cover gray hair effectively. When dealing with a high percentage of gray hair, the goal is to achieve uniform coverage and color durability, which temporary dyes cannot provide.
Semi-permanent color might seem like an option as it lasts longer than temporary dye, typically four to six weeks, and can create more natural-looking color variations. This might look more natural and blend some grays, but may not be opaque enough for hair that is predominantly gray. Semi-permanent dyes also often fail to fully pigment white or gray hair, which can lead to inconsistent color and less satisfaction with the results.
For those wishing to delay the move to 100% permanent color, using a semi-permanent dye as a base with permanent highlights can be a technique to merge the natural grays with the dyed hair for a more blended appearance. However, for overall maximum coverage, a permanent color is generally the recommended route.