Rug Over Dye
Rug over-dyes are simply choosing a (one color) color and dying over all existing colors. Adding one color to existing colors will change all colors or darken or refresh existing present colors. If original rug color is one color then the over-dye color will be determined by original color and new color chosen. You could in all practicality consider an over-dye a color change but color changes normally just deal with one color to another.