This is a simple Tutorial to make Rainbow effect in Photoshop, its much easy i mean dead easy...
Our final image can be like this
Now lets start the tutorial. Take the image which you want to use... i took the image below...

Now lets put some Noise on it. Oh stop, not the human Noise i meant Photoshop noise :D
First of all duplicate this layer as i told you in other tutorial Make 3D heart in Photoshop. or simply right click on the layer and click on "Duplicate Layer"
Now go to Filters > Noise > Add Noise
Put Noise settings as shown in the image and press OK
Now its time to put some Motion Blur on it,  go to Filters > Blur > Motion Blur
set the Blur settings as shown in image
Now you have two Layers, one is original and 2nd is with Blur and Noise filter. Now set the Layer Style to Lighten as shown in image.
Now you image is ready, this is instant rain effect in photoshop. I'll put another tutorial on it in little bit advanced way. :)