The Blend Tool is really useful for creating smooth color transitions. Also, it’s great for creating very line intensive designs. The butterfly and flower above were created using the Blend Tool.
For the flower, I simply drew an arch with the Pen tool and used the Reflect Tool to copy it to the other side. Then I changed the color of one of the sides and made sure that they met evenly at both corners. If you can’t get the ends to meet perfectly, just get one end to meet evenly and make sure that the other one isn’t too far off.
Then with both sides selected, double click on the Blend Tool to open up a pop up window. Select the Specified Step option and then pick how many steps you want in between the two already formed arches. Then it should create those little intermediate arches. Now simply copy and paste however many petals you want for your flower.
The butterfly was actually created by creating an oval and then using the Anchor Point Tool to turn your oval into more of an eye shape. Copy and paste the eye shape. Then I made sure that there was a Stroke/Border on my eye shapes and no fill. Now use the Blend Tool with the Specified Step option still selected and it should create kind of a weird caved in look, the intensity of the blend depends on how close the shapes are together. Since I was satisfied with my shape blend, I moved on and made duplicates of the shape with the Reflect Tool and then copying and pasting the first two shapes.