Inkscape is a Vector Graphics Editor, similar to Adobe Illustrator, that strives to be SVG Compliant, open source, responsive and extensible.
Below is my result and I'm happy with it.
CREATING THE FULL SPECTRUM CIRCLE IN INKSCAPE.
1. Create circle in Inkscape
2.Select“unset” fill by right-clicking on the fill color.Low the opacity to 40.
3.Select the circle and click on it again. You will see the rotation mark. Drag the rotation of the center downward the circle.
4.Go to Edit > Clone > Created Tiled Clone.
5.Tiled Clones Value
6.Click the create button and you will have a full spectrum circle!
7.Create a black rectangle and put it behind the spectrum circle. Save the image.
POLISHING THE FULL SPECTRUM CIRCLE IN GIMP.
Import the full spectrum circle image in Gimp
Go to Filters> Enhance > Unsharp Mask. I use 0.1 for the radius,6.0 for the amount and O for threshold value.
Next, duplicate the layer
Go to Filters> Enhance > Sharpen.
I use 40 for the values. Then flatten the image. Finish.
POLISHING THE FULL SPECTRUM CIRCLE IN PHOTOSHOP.
This is my other experiment. Using Photoshop.
Import the spectrum circle image in potoshop
I use Filter> Artistic> Paint Daubs.
Finish. Adding some simple text.
Result in Photoshop