What does "muted" mean in the context of color? I want to extract colors from images like in Adobe Kuler. When I select colorful or muted corresponding colors are shown. But what is the definition for colorful or muted colors? What do the words "colorful", "deep" and "muted" refer to?
In terms of the HSV color space, "muted" colors are those with lower saturation and/or value.

"Deep" colors are saturated but not too high in value (e.g. deep red) while colors with both high saturation and value might be called "bright" (e.g. bright red).

"Colorful" is somewhat more vague; it might mean bright colors, but it also might mean having a range of different hues within an image or palette, such as including complementary colors or analogous colors rather than just variations on a single hue.

Note that these are general terms that would be understood by any graphic designer or artist, but the specific algorithms that Adobe Kuler attaches to these words are proprietary and only known to Adobe.