Have plenty of frosting on hand to practice. To blend colors, first tint a small amount of buttercream a dab at a time. Blend after each addition until the color is darker than you would like. (You can use a single color or experiment with mixing.) Gradually mix the tinted frosting into more buttercream until you get the right shade. Pipe flowers onto parchment or waxed paper until you're ready to try them on the actual cake.