Final answer:
Permanent hair color is the best choice for covering hair that is 30-50% gray, as it changes the hair's natural pigment uniformly, offering more complete coverage than semi-permanent, demi-permanent, or temporary colors.
Step-by-step explanation:
The color that provides gray coverage for hair that is 30-50% gray is permanent color. Semi-permanent color can blend some gray hair, making it less noticeable, but it will not cover gray hair uniformly. Permanent hair color alters the hair's natural pigment, allowing for a uniform color that can cover gray hairs effectively.
Permanent hair color involves a chemical process that removes the natural color and replaces it with synthetic color. This type of color lasts until the hair is cut, although the synthetic color may fade over time. Since semi-permanent hair color cannot lighten hair or cover gray completely, and it washes out after four to six weeks, it would not be the best option for covering 30-50% gray hair.