Create a Rainbow in Photoshop

Run Polar Coordinates Filter

7. Go to Filter > Distort > Polar Coordinates. Make sure "Rectangular to Polar" is selected. Click the - (minus) button in the filter dialog if you want to preview the effect.

8. Click OK and your document should look like the example here.

Index: Create a Rainbow in Photoshop

1. Set Up the Document
2. Add a Gradient
3. Run Polar Coordinates Filter
4. Finishing the Rainbow