Many substances will dye wool various shades of brown, tan, and buff. Fewer dyes are available for reds and yellows and still fewer for blues and greens. Purple was the rarest of all. Tyrian Purple was extracted by the Tyrians from a local shellfish. 8500 shellfish were needed to produce 1 gram of dye, making this a color to be afforded only by kings and princes.
Some dyes tend to fade over time while others remain vibrant for a long period.
A dye that does not fade is called lightfast . One of the most popular dyes of all times, indigo , is not particularly lightfast. You are familiar with this dye as it is the blue in blue jeans. As you know, blue jeans fade over time. This has been a disadvantage of indigo for millenia.