I used leftover natural blue food coloring (made with red cabbage and baking soda) to make some print samples. I used a fork to print mini polka dots and a pattern which looks like rain. The color was dark green before I washed the fabric.
The dye isn’t very washfast, it bled while washing the fabric with lukewarm water. Once the fabric is dry, the color is light teal.
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What an interesting experiment. Not ideal for something that needs washing but it might be fun to make some art work maybe.
You can make a washfast and lightfast blue dye with red cabbage if the fabric is mordanted with iron liquor.