This is a neat trick that will give you some really interesting and extremely colorful results. To start off, create a new document. It can be any dimension that you want, and click ok. Mine is set at 1024 x 768. Then, create a new layer and grab the gradient tool. Go up to the gradient preset and choose the yellow, violet, orange, and blue gradient. The key to this trick is to set the blend mode of the gradient itself to difference.
With your gradient tool still selected, click and drag multiple times across your canvas. Be leery of creating obvious bands of colors, but if it starts to get that way, just change the directions in which you are clicking and dragging. In other words, click and drag horizontal, then vertical, and then diagonal (mix it up a bit). The image itself may begin to become dark, which may not be what you want. If this happens, simply Hit Command + I (Ctrl + I on the PC) and the entire image will invert, and you will get a fluorescent sea-foam green type image, and you can begin clicking and dragging again. You can do this an infinite number of times, so when it becomes too dark, invert it. I almost recommend it, because it will add real depth to your colors and impact to the overall image.
If you find that your colors have become a little muddy, that isn’t really a problem either. Go to your Adjustments panel and create a vibrance adjustment layer, and bump up the saturation value to around 10. Any higher, and you will lose some detail, and the colors will start to become blown out in areas. For the vibrance slider, I bumped it all the way up. You can do it to taste, because it really depends on your image. Then hit Command + E (Ctrl + E on the PC) to merge the adjustment layer with your psychedelic layer.
I like the effect that we have now, but if you want a more fluid look, then you can go to Filter> Liquify and using a large brush, you can swirl a few of the colors around to create a melted effect. It is really about taste, and for my purposes, I am leaving mine as the cloudy look.