Color shift yarns are created to be series of colors that work together in a project. The yarn is intentionally dyed so that colors within a set coordinate. A "color shift" is when the colors in the series move or shift from one color to another gradually, the intermediate colors are all related to the two end colors and therefore coordinate. A "tonal shift" is light to dark shades of one color. Most of the colors are also available in whole skeins on most bases.
Pattern Inspiration for Gradients & Fades (on Rav)
Meridian Gradient Set Ravelry Project Gallery