I'd probably also use the pen tool, but here's another way that almost, but not quite, gives the desired result.
On a new layer, create several colored bands:
Make a rectangular selection of the bottom half of your bands, then go to Edit>Transform>Perspective and widen the bottom edge. In your original image, the transitions are curved, but in my version they are sharp corners. So it's not 100% the same, but it's pretty close.