This is called bag striping or brush striping. When you affix the rose tip to the bag, line up the narrow side with the stripe. As the petals are squeezed out, their edges will be tinted. It's easy and looks spectacular. When you've applied all the flowers you want, add a leaf or two to each one, including drop flowers.
Leaves make the cake look finished.
Using a bag of green icing, affix a leaf tip and make small leaves with one of the pointy sides of the tip facing down. As you finish the leaf, lift up so the tip sticks up. This makes it look more realistic. This will ensure that the tip of the leaf comes to a nice point without breaking off. This same trick works for icing to be used in writing.