The following plants work very well:
 

• green colouring: spinach, parsley, St. John's wort, red cabbage (produces turquoise colour)   
• blue colouring: blueberries                    
• yellow colouring: camomile, saffron, apple tree leaves                                                
• orange colouring:  carrots
• red colouring: beetroot
• brown colouring: red onion skins, coffee, tea

Make a dye out of the different plants. Break up or grate larger parts of plants, or follow the following rules of thumb for the various different ingredients: 
 

• vegetables: 500g in 2 litres of water, no soaking necessary, boil for 35 minutes
• flowers, leaves and berries: 30 to 100g in 2 litres of water, leave to soak for a few hours, boil for 35 to 60 minutes
• coffee and tea: 30 to 50g in 2 litres of water, no soaking necessary, boil for 25 to 30 minutes

Hard-boil the eggs. Eggs with a white shell are better, as the colour produced using natural materials is never as strong as when using artificial materials. Colouring eggs is best done with eggs from free-range hens – the shell is harder, the egg healthier and it's a better choice in general. You'll find quality-controlled free-range eggs from 'Red Rooster' on these farms.

Next, place the eggs in the ready-prepared dye for around half an hour. Turn the eggs frequently so that the colour is not just on one side. You can leave them for a shorter length of time too, but the colour will not be so intense.
 
Finally, leave the eggs to dry on a tea towel and rub with bacon rind to give them a nice sheen.
 
If you'd like to decorate your eggs, use a sharp object to scratch the surface. This will simply scratch away the top layer of colour.
 
Tip:

You can use parsley, lemon balm or ivy to make great decorations. Simply moisten the leaves and place on the egg shell. Put a pair of tights over the eggs and leaves, bind tightly (so that the leaves stay in place) and put in the dye.