When thinking about natural dyes and pigments there are a couple of things that you should remember.

Trade among the indigenous peoples of No. America was continent wide long before the arrival of Europeans. It was not uncommon to have access to goods from almost anywhere on the continent. A dye or pigment, that worked, was traded and traded again. So don't limit your choices to just your local area.

To create dyes and pigments you can't think just plants. Many, if not most dyes and pigments are from minerals. Ocher is Iron oxide and is found in yellow, red, and sometimes blue.
Minerals are ground and mixed with such things as fat, grease, and pine sap.

And the native peoples were not adverse to mixing things that we think of a nasty. For example a common way to make black dye is a mixture of Willow bark and urine. In fact urine is a very good mordant. It is also used to treat fibers getting them ready to take a dye.
(If you last name is Fuller it may mean that your ancestors may have treated Wool by walking barefoot on it in tubs filled with urine)

These are only a few examples but the idea I'm trying to get across is that you shouldn't limit yourself to your local plants when searching for Tartan colors.