Before synthetic ink, ink was primarily made from plant dyes, which you can still make today. Different plants produce different dye and ink colors, so be sure to choose a plant that produces a color you like before you make your ink. Other ingredients, like vinegar and arabic gum, should be added to most inks to make the compound last and stabilize.
You can use a variety of different wood barks and wood hearts -- the inside of the tree -- to create brown, black or red inks. Oak bark, the inner bark of the white birch and the bark of the Colorado fir all create tan to dark brown ink once crushed and added to water or vinegar. The inside or heart of the logwood or Brazilwood creates a deep reddish ink. To create an ink, you can add crushed pieces of vinegar or water and let them sit until the color infuses the liquid, or you can boil the pieces in the liquid for a richer color and faster results.
For a fun, safe ink anyone can use, crush a few berries. Strawberries, raspberries, blueberries and a variety of other berry fruits are often used to create a wide range of ink colors, including red or blue. To make a berry ink, crush a 1/2 cup of berries in a strainer over a bowl. Crush the berries until only the pulp remains. When you're done, add a 1/2 tsp. of vinegar and a 1/2 tsp. of salt to retain the color and keep the ink fresh. Berry inks should always be kept covered and sealed in a jar to keep them from growing mold or turning rancid.
For more unusual colors, try flowers. Use hyacinth flowers to make a blue ink. Crush rose, lavender or hibiscus petals for a red or pink ink. Lilies of the valley, artemisa and snapdragons can all be used to create a wide range of greens from a pale shade to a darker forest shade. Marigolds, daffodils and dandelion create a bold yellow ink. Steep the flowers in boiling vinegar or let them sit for a while in the liquid to obtain the color you want.