This image looks like it used both the pen and circle tools to create this flower, with a lot of gradient coloring used to fill it in. the color swatch palette might have been used in the center of the flower. The leaves and flower petals look to have a yellow stroke around them to to separate the flower from the white space. The image can be completed in illustrator, But coloring looks lIke it can be done in Photoshop as well.
The illustrator of this design most likely focused mainly on the pen tool, and the shape tool, creating numerous shapes throughout the illustration to create the final product of a beautiful flower. The yellow stroke around the leaves and flower petals creates a contrast that really works in this illustration, almost using the yellow as white space to separate the pedals from the leaves. My knowledge of gradient coloring is fairly minimal, but I assume that there was definitely some of this coloring used to get the faded effect that works to well to establish dimension and depth in this illustration.
This image was probably fairly easy to create in Illustrator. The designer only needed to create one pedal and one leaf, then copy/paste the image multiple times to create the whole picture. The pedal was probably made using the ellipse tool. The pink/purple ellipse was transformed using the direct selection tool to get the teardrop-like shape. The leaves were also made with the ellipse tool, and then transformed to look like a leaf. Some effects were probably used along the edges as well. Everything was filled in using the gradient tool, because the leaves and pedals look faded. The ellipse tool also created the circles in the middle of the flower.