You can start with a multi colored sample gradient like this:
Then click on the colors of the gradient in the option bar, so you can work in the Gradient Editor.
Click on a Color Stop and, with te color picker, pick up a color in your sample image (make sure you open this image in Photoshop). Do the same with the other Color Stops. You can make further adjustments by dragging the stops, adding new ones of removing them.
Click New to make a new Gradient Preset. Now you can use your Gradient.
Finally draw the circles on a new Layer and lower the opacity of this Layer.
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