Firstly, wts is a mordant? It's a powder, liquid, whatever, that helps the dye stick to whatever you're attempting to dye. Most natural fibers need something to help the dye stick. But some natural dyes don't need a mordant, so you can skip that step.
These dyestuffs can work well on their lonesome:
* walnut hulls and some seaweed turn something blackish (dark brownish and dark greenish)
* coffee grounds and avocado skins can give you a brown
* Red beets and saffron, if you want to waste your saffron on dye, can give you reds.
* Tumeric can give you nice yellow
* Blackberries can give you a blue color.
You can mix and match, get a purple hue with a detroit red beets and blackberry combo. Chop your stuff up, put it in a large pot, cover it with several inches of water. Soak the walnut hulls and avocado skins overnight. Anything more solid or tough than herbs, boil for about an hour. Herbs take about half that time.
After boiling, strain the solids out of your dyebath. Let it cool. Add just enough water so your fabric or yarn can move around freely. Boil the dye and your fabric or yarn for about an hour. Rinse, and carefully dry, and continue crafting.
Sometime in the future, I'm gonna test hopi red dye amaranth and see if we can do it sans mordant.