I've been using the tartan generator @ www.tartansauthority.com (link) to design tartans. I assume the colors in its palette are standard tartan colors.

I was just curious to know how much more expensive a tartan would be if it included one custom color. For example, the tartan I designed for Arizona features a color matching the copper-colored star on the state flag.

I assume custom colors make little or no difference when working with printed colors (on T-shirts, for example), but are a bigger factor on woven materials.