I chose blue because it was for a guy's birthday but you can do any colour you like, or even a single colour, but the petal making remains the same.

What you need:
Butter Cream Frosting (find recipe here)
Piping bags (I prefer disposable)
Small angled spatula or small spoon
a cake

What to do:
Start with your white butter cream frosting, depending on how many colours you're using put some white frosting into a piping bag (e.g. if you are using 5 colours, place 1/5th of the frosting into the bag). 

Next, decide what colour you're using, I used blue so drop a few drops of food colouring into the frosting and mix. Once you get the desired colour, place (now 1/4, because there are 4 colours left) into a new piping bag. 

Add a few more drops of food colouring to get a darker colour and place (now 1/3rd) into another piping bag. Continue with this method until you're done with the colours and frosting. I did two rows of the darkest blue so I doubled up on that frosting in my last bag. 

Now you should have 4 (or how ever many) bags of frosting. I like to use disposable ones so I can just cut the tip off the bag instead of using a tip. If you are using a tip you want use a large round one, like a Wilton #12 tip (but remember you'll need at least 4 of them).

Now it's time to start frosting, make sure your cake has a crumb coat to avoid getting any crumbs in your petals. Always start from the bottom and work your way up. Start with your darkest colour and place a dab of frosting (penny sized) onto the cake. With your next darkest colour place another dab directly above that one, and so on, until you reach the top.

Using a small angled spatula, or the back of a spoon (spatula gives you a much cleaner looks, press down on the frosting blob and in a single motion swipe to the right to flatten 3/4ths of the frosting. Continue with the other frosting blobs until you're done. Cleaning your spatula after each swipe, you must do this for 2 reasons, first you don' t to mix colours and secondly because you will get a cleaner looking petal without extra frosting being transferred on. 

Starting again from the bottom and your darkest colour place another dab of frosting just a little to the right of the previous one. Continue the previous steps by placing another dab directly above that one and so on. Again, flatten 3/4ths of the frosting blob by pressing down and swiping to the right. 

Continue this all the way around the cake. 

As you get all the way around the cake, there will be a seam in the back. When you reach the last little area instead of swiping to the right you will leave it as a dab to finish off. 

For the top of the cake, start where you began the petals on the side of your cake, so your seams will line up, and do the same process again. Dab a small amount of frosting, and swipe to the right, place another dab beside that one and swipe again. This time, instead of going vertically we're moving in a circular motion. 

You can continue with the ombre look and use a darker colour as you move toward the centre or you can fill the centre with filling, like I have with this dulce de leche filling. Both ways look great.