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Dissolve coffee in water, paint on. Different concentrations of coffee will give lighter or darker shades. I have tried a variant of this, soaking wood in instant coffee, and it worked.

Suggestions for using natural foodstuff to make dyes to stain wood. Not just coffee or tea, but spices (turmeric, paprika), berries (raspberries, blackberries), leaves (spinach, violet). This also notes that such dyes are not colourfast, and then suggests a beeswax + oil finish.