Final answer:
Semi-permanent hair color cannot lighten hair. It washes out over time and may not dye gray or white hair to the same color as the rest of the head. Using semi-permanent as a base with permanent highlights can delay the need for 100% permanent color.
Step-by-step explanation:
Semi-permanent hair color cannot lighten hair. Hair can only be lightened using chemical lighteners, such as bleach. Bleaching is always permanent because it removes the natural pigment.
Semi-permanent color washes out over a period of time-typically four to six weeks, so root regrowth is less noticeable. The final color of each strand is affected by its original color and porosity, so there will be subtle variations in color across the head-more natural and less harsh than a permanent dye. However, this means that gray and white hair will not dye to the same color as the rest of the head (in fact, some white hair will not absorb the color at all). A few gray and white hairs will blend in sufficiently not to be noticeable, but as they become more widespread, there will come a point where a semi-permanent alone will not be enough. The move to 100% permanent color can be delayed by using a semi-permanent as a base color, with permanent highlights.