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.
You know, that doesn't really look like a mask on a background to me; it looks more like a gradient mesh created in a vector drawing program like Adobe Illustrator.
Now for the bad news: GIMP, being a raster graphics editor, doesn't have gradient meshes. Inkscape doesn't have them either, although it's apparently being worked on. If you're feeling adventurous, you can enable the experimental mesh gradient support in Inkscape by recompiling it from source and following these instructions.
There is another easy way to achieve this result, but it is only relevant if you don't want to exactly rebuild the same blur.
Open the image in your favorite image editor, for example GIMP, and simply apply a heavy Gaussian blur. The advantage of this approach is that you have nice color mix in the resulting image (depending on your input image).
The quickest way is to use a site like design-seeds.com, specify the color you're aiming for and then using one of the images there. Zoom in massively on a promising section, take a screen shot, import and apply blur, filters, lower opacity at will. Copyright issues are imaginary here because it's unrecognizable and untraceable and you're hurting nobody. If you want to be a saint peruse your own photo library or take a picture for that purpose on full zoom and out of focus if you have something with the desired colors in your vicinity.