Here goes the trick. I wanted one of my rainbows to 'dissolve' into separate lines, each color flying in separate direction. Here is how to do it: for each layer with color, select the shape with Path selection tool(A), then switch to Pen (P) or Rectangle (U) (does not really matter), change shape mode to Substract, and add a shape that hides part of the circle. Repeat with each color of rainbow - it will look nicer if lines are cut in different locations.
Now we need to add 'flying' lines of colors. We could do that with Pen tool only, but unfortunately Photoshop does not have curved lines with fixed width, or path offset (as Illustrator). And I wanted the lines to have fixed stroke width. So we will have to add paths and then transform them into pixels. So, open Paths palette (from Windows menu), switch to Pen tool, then on bottom of paths palette, click New button. New path will appear (called Path 1).
Start your path by clicking exactly the middle of where red line is cut, and dragging (and releasing) the mouse continuing red line direction.
Continue your path until it is outside document boundaries.
to Brush tool (B), set forecolor to the same red
color as the rainbow red, then carefully choose brush size that matches size of
rainbow line (in my case, 3px). Then, hit Stroke button on bottom of Paths
palette (2nd in the row). You can click Stroke button several times
to make line
wider. If line does not match, undo and change brush size. You should be careful
and patient doing that :-)
Next step is to blur edge of line, to add some depth to our artwork. Turn on Quick Mask (Q), then switch to Gradient tool (G), select black-to-white gradient, and apply gradient as shown here.
Turn off quick mask (hit Q again): you should see the selection similar to this.
Go to Filters menu - Blur - Gaussian blur, and apply 3-5 pixels blur. You can also try Lens blur - it may look better (or not) but take much more time to apply.
Now the red line is completed! Repeat the same steps for each color.
Next step: in new layer, add some very small spots with white hard small brush (add some 1px dots, then 2px and 3px)