Answer: Usually the plant part isn't the same color as the dye it produces. The plants also need to be mixed with a "mordant," an agent that helps "fix" the dye color. Typical mordants are alum or cream of tartar. There are many herbal dye books available. I suggest that your son gets one from the library as it will be useful. Here are a few color ideas:
onion skins, orange; marigold petals, pale yellow; chamomile (Anthemis tinctoria) flowers, bright yellow; heather (Calluna vulgaris) fresh branches, green; madder (Rubia tinctorum) roots, rich tomato red. Mixing the colors doesn't work as well as mixing paints, for example!
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