Similar to Mxple_Starz 3rd suggestion, all contestants will be given a theme to follow. For example, say the theme is "Underwater", then everyone will make a skin with the palette following the underwater theme. This allows contestants to have a bit of direction when making a skin and when poll time comes, the voters will have another thing to take into consideration (another idea that may be too tough, but the palette could also be made to contrast the theme. So the theme would be "Underwater", but the palette will have very few colors that relate to underwater and will instead have warmer colors).
This can also be taken further and instead of a theme, a single word will be given and contestants must design a skin based off that word. For example, lets say "Memory" is the word, so all contestants using the given palette will create a skin with memory in mind. They can interpret "Memory" however they wish. The words shouldn't be too specific (maybe once in a while) and should be somewhat vague.