Found this site for henna info. Here's part of the info.
http://hennabycynthia.homestead.com/FAQ.html
[FONT=Helvetica, Arial, sans-serif]Special Recipes and Hints
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[FONT=Helvetica, Arial, sans-serif]Coffee
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[FONT=Helvetica, Arial, sans-serif]To help tone down red tones, deepen brown-red tones, or cover grey, substitute coffee for water. Used brewed, black coffee (not instant or decaffeinated). After coffee is brewed, bring to a boil. Mix as directed.
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[FONT=Helvetica, Arial, sans-serif]Teas
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[FONT=Helvetica, Arial, sans-serif]Ceylon or Black China Tea will add gold highlights to Light Brown Henna. Red Zinger Tea enriches red tones in Burgundy, Sherry, Mahogany and Red Henna. Chamomile Tea brightens and adds highlights to Neutral, Blonde or Marigold Blonde Henna. Let tea steep for 20-30 minutes. Bring to a boil. Substitute tea for water. Mix as directed.
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[FONT=Helvetica, Arial, sans-serif]Apple Cider Vinega[/FONT][FONT=Helvetica, Arial, sans-serif]r
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[FONT=Helvetica, Arial, sans-serif]To help color hold on grey hair, add 2 tbsp apple cider vinegar to Henna mixture.
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[FONT=Helvetica, Arial, sans-serif]Cinnamon
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[FONT=Helvetica, Arial, sans-serif]To brighten and highlight red tones, add 2 tbsp cinnamon.
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[FONT=Helvetica, Arial, sans-serif]Lemon Juice
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[FONT=Helvetica, Arial, sans-serif]To lighten blonde shades, add 2 tbsp (or more) lemon juice to Henna mixture.
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[FONT=Helvetica, Arial, sans-serif]To Deepen Red Shades
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[FONT=Helvetica, Arial, sans-serif]Red, Sherry, Mahogany or Burgundy can be deepened by mixing a darker shade. For example, 3 oz Mahogany + 1 oz Dark Brown = Dark Auburn.
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[FONT=Helvetica, Arial, sans-serif]Dry Hair
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Add 1 egg, 2-4 tbsp olive oil, or 2-4 tbsp plain yogurt to Henna mixture. These natural ingredients not only condition, but help with the application and rinsing processes.[/FONT]