how to dye cob oven plaster?

Hi all, i'm just about ready for final plaster for my cob oven. I want to aim for 'purple', currently it's reddish gray (1/3 horse manure, the rest appropriate clay/sand/water ratios).
What is a basic, easy to find dye/pigment I can get (wildcraft) for free/cheap in NW? I added some blackberries (on the ground ) into the mix just for fun to see if they would help. No go. worked great for the wooden blast door. this is a double chamber, hard to see smokestack opening on top.

thoughts welcome. Thanks

It's not free, but you local home center will have lots of suitable dye in the tile/grout aisle and again with the bags of cement and concrete block, and it is fairly cheap. There is less of a selection of colors in the cement aisle, and what they do have is more earth tones and not purple, so you might want to stick with the grout coloring agents. Since you are not going to be following the label on the use of the dye, it's going to take a little experimentation on your part to find out how much you have to use to get the color you want. I would also advise against mixing dyes thinking you are going to get a color that is halfway in between. When you mix two strongly absorbing distinct colors, you spread out the spectral absorption and all that gives you is 'brown'. Brown is not a color of the spectrum as are red, purple and orange. Rather it is the broad absorption of red, purple and orange all mixed together. So find the shade of violet (another true color) that you like and see how much of that you have to add to your red/gray starting material to dry to the color you want.