Final answer:
Demi-permanent hair color is not ideal for gray and white hair, as it may not dye these hairs to the same color as the rest of the head.
Step-by-step explanation:
Demi-permanent hair color is ideal for all but gray and white hair. While semi-permanent color washes out over time and is less noticeable on root regrowth, it may not dye gray and white hair to the same color as the rest of the head. Some white hair may not absorb the color at all. When gray and white hairs become more widespread, a semi-permanent color alone may not be enough, and a move to 100% permanent color is typically needed.