I have seen an image with this background. As a newbie I am unable to get this multi colored mixed background. I am able to get just two colors and that too linearly with the Blend Tool. But that doesn't look like this at all. Can some one please suggest me on how to achieve this in GIMP or in Photoshop?
There are many techniques to do this. One very simple one that I often use in practice is to use many colors on a similar palette and just generally paint them using the brush tool all over the place. It looks incredibly bad at first, until you apply the Filter→Blur→Gaussian Blur to taste.
The end results is a very simple, diffused, gradient-like, background. This is also accomplished in Adobe Illustrator through gradient meshes, but is a more complicated and precise process.
Note: If you're wondering how the edges were not blurred, I contained them with an unlinked layer mask that was not affected by the blur.