You could get a pen designed for fabric and highlite it.

Riley Blake has some cotton ombre fabrics available, there's a nice yellow:

http://www.fabricshack.com/cgi-bin/S...es=0&lastmenu=

I would Blanket Stitch or Satin Stitch with Black Thread that would make it pop!! Love the fabric and the applique

O.K. silly me ...what is thread painting?

I would do some test with the pens used for making labels. See if you can outline the edge as done on the fabric. Variations in fabric color would also help.

www.craftsy.com/article/thread-painting
Thread painting is a technique that can help you achieve realistic effects on art quilts and portrait quilts. As you would expect from its name, thread painting by.............

Paint sticks, fabric paint all will do a great job and then add a grey shadow on the background To make it look raised from the background. I

I saw a picture of a quilted sunflower with beads to make it stand out.

Black satin stitching should help it pop and if you really want to do more in the center, either thread paint the shadow in deeper yellow or use a pigma pen. Very pretty!

I'd use Paintstix or Derwent colored pencils. For the Paintstix, use a stencil brush to scrub the paint onto the fabric where you want it a darker yellow. You can blend w/ the stencil brush so the edges won't be a sharp line. Derwent colored pencils (many YouTube videos) can be applied a couple different ways. You use a fabric medium or clear aloe vera gel to make the color flow and it will also make the color change to very vibrant. Heat set with an iron after it's dry. You can put the fabric medium or clear aloe vera gel on first and then color into it or the other way around. The pencils can be blended some by just layering one over another. Try it out first on some scraps.

I have permanent fabric markers that work great and after being heat set are very washable. I would use these to add dimension to the flower. I have also used them to blend across a seam in a landscape fabric when the two sections didn't line up just right.