Jill - from Calgary, AB
Below is a link to a beautiful ombre cake from Wilton.com. Although this is a 6 inch cake, I thought it would help give you an idea of how much icing you would need. For a 9 inch ombre cake, I would suggest making at least 3 cups of icing for each color (so 9 cups total). It's always better to have too much icing colored than not enough.