It works the other way around. You choose which part of the outfit you want to dye, then you're given a palette like this:
[Image: http://wiki.mabinogiworld.com/images/thumb/c/c8/Cloth_Dye_Palette.png/200px-Cloth_Dye_Palette.png]
A cursor is overlayed on top of it and it gives 5 different spots on that palette as "previews" as to what colour you could get. So for example, you might get 1 white, one blue, and 3 puke green previews.
After clicking while your mouse is on that cursor, it shows you what each of these 5 previews will look like on the outfit.
If you select "start dyeing" after that, you then have a 20% chance of getting one of the given colours.