A skirt that has at least two or more colors present in its design.
Take care when there is an image with a layered skirt in it though, such as post #1850259. If each layer has a single color, then it isn't a multicolored skirt. Each layer can be tagged with each individual color, so for the aforementioned post, the tags would be layered_skirt yellow_skirt red_skirt blue_skirt. This tag is specifically when a single layer of short cloth has multiple colors on it.
See multicolored clothes for guidelines on what is considered multicolored.