I found this fun diy – project on Free People Blog. An experiment with natural dyes! You can for example use red-cabbage for a bluish/purple dye. Blueberries can make a lovely dusty rose color and yellow onions can bring out an alluring mustard yellow. When you work with natural dyes, remember that experimentation is the key.  Depending on the amount of ingredients you use and how long you leave them in the water, the color you get can vary. I prefer ingredients you can still eat afterwards or you can use ingredients that aren’t so eatable anymore..

And thats how it works: Pictures and instruction from Free People Blog

• chop up your ingredients and put them in a pot with twice as much water as ingredients.  Bring the water to a boil and let simmer for an hour.  For deeper colors, you can leave the ingredients in the water (without heat) overnight. Remove the hard materials from the mixture with a strainer, leaving you with the liquid dye.
• Before dyeing, you will also need to create a fixative, which will help your fabric hold the dye. When working with berries, use a salt fixative – put 1/2 cup of salt in 8 cups of water, put the fabric in and boil for one hour. When working with vegetables, use a vinegar fixative – mix one part vinegar and four parts water, add your fabric and boil for one hour.
• When you remove the fabric, rinse in cold water.
• The fabric is now ready to dye! Just drop it in the desired color, let sit until it reaches the shade you want, remove and hang to dry.