We have to represent now a rainbow on a new layer, using the Brush Tool.



Blur it out also: Filter - Blur - Radial Blur.



Turn it vertically: Edit - Transform - Flip Vertical and change the layer's type on Linear Dodge. We have to distort it also, using the combination Edit - Transform - Distort. The layer's Opacity should be situated on 50%.



Pass on the rain's layer and correct the layer's colors (Selective Color).



Merge down all the layers: Layer - Merge Visible (shift+ctrl+e) and now we have to apply the filter indicated here: Filter - Render - Lens Flare.