Premium Colours would then be colours which have special effects to them, with some blanket examples below.
Metallic
Metallic colours have a "texture" to them, appearing very, very finely granulated and ever so gently glimmering when it catches light the right way. The effect shouldn't be too blatant though, it should look nice without being a disadvantage to those who decide to use it.
EXAMPLE
Pearlescent
These paints are ones that, depending on the amount of light and the direction the light is hitting it, changes colours. For example, a purple pearlescent colour could look fairly normal in darker conditions, but when it is light out the purple turns into a medium powder blue depending on how the light hits it. Like metallic colours, the effect should be nice but not so glaring that it acts as a disadvantage just by taking it.
EXAMPLE
Of course there are more types of specialty colours out there (feel free to suggest your own), Metallic and Pearlescent are just two examples.
EDIT NOTE: Added some examples for the Metallic and Pearlescent colours.
Edited by Joseph Marik, 25 April 2013 - 03:30 PM.