Two tone icing can look very good on cupcakes and birthday cakes. When subtle it is pretty and petal like, when bold it can be very eye-catching.
You will find different suggestions on the internet as to how to make 2 tone icing but this is the method I use to produce the effects in pictures 1, 2 and 4.
Make up your white buttercream icing then put a couple of spoonfuls of the icing into a separate bowl and using gel colourings make the icing a very deep colour. Put a spoonful of the icing into your icing bag with nozzle fitted and on a clean surface flatten the icing in the bag as shown below.
Shake the bag open and fill with the white buttercream icing and then pipe onto your cupcakes or birthday cake. A deeper more pronounced edge colour will be obtained by putting more of the original dark buttercream into the icing bag initially.
If you reverse the colours ie. put the lighter colour into the bag before flattening the bag and then add the darker icing you will get an effect similar to picture 3 above.