Buttercream Frosting/ Icing is a type of icing or filling, used inside cakes, as a coating, and as decoration. In its simplest form, it is made by creaming butter with powdered sugar, although other fats can be used, such as margarine or lard. Colorings and flavorings are often added, such as chocolate, fruit purees or various extract. Buttercream is a common and popular topping for cupcakes, sponge cakes, butter cakes and other desserts. (Courtesy: Wikipedia)
I wanted to give a different look to the Icing instead of usual single colour. I brushed the inside of Using different colours and a thin paint brush, I made stripes inside the piping bag. Tipped in the Icing and Voila!!!
Buttercream Frosting
Source: Joy of Baking
Ingredients
Icing or powdered sugar- 2 cups
Unsalted butter- 1/2 cup
Vanilla extract- 1 teaspoon
Milk or light cream- 2 tablespoons
Colours- of your choice
Method
Cream the butter and sugar till smooth and well blended. Add vanilla. With mixer on low speed, gradually beat in sugar. Add milk and beat on high speed till light and fluffy. Add a little more milk, if needed. Use colours as per your choice.
For Rainbow icing, take the piping bag, your choice of colours and a thin paint brush. Carefully dip the brush in a colour and draw a line inside the piping bag, starting from the tip. Wash the brush and dip in another colour. Leaving a small gap, draw another line. Continue till you are finished with the colours. Carefully put the icing inside the bag. Squeeze out a bit to check if it forms the stripes. Start decorating your cake!!! 
Dear Mareena,
Thanks and great job…..This is an interesting icing…but to tell you honestly…If there wud be step by step explanation with pics…it wud help a lot……three cheers for ur wonderful recipes.
thanks and best wishes
liz
Good point Liz. Will try and click pics the next time I make them. The issue I am facing now is that I dont have anybody to click the pics while I am working on it..Anyway wil try to get around that problem as soon as I can..Thanks for your kind words and suggestion…Truly appreciate it..
In the piping bag do you draw coloured lines horizontally or vertically?
If you are holding the piping bag with the tip pointing to the ground, draw vertical lines starting from the tip towards you. Hope you understood. Please feel free if you need more clarifications..