Gautam, here's another similar way to do this type of effect.
You first need some corner brushes that you like and want to use.
Here, I have used two separate brushes to make this design. They are black for the moment. I place each one on it's own layer. Then I turn off the layers.......I do this because I'm only going to use then as templates.
I create a new layer.
I
Comnd/Cntrl + click the thumbnail of the first corner template. This makes a selection in the same shape.
I then use my
Brush Tool and a soft round brush, to brush in different colors. These colors will remain inside the selection.
I do the same for the other corner template.
On a new layer, use the
Brush Tool and a soft round brush, to brush in different colors.
I add the same layer style on both colored design layers to add a drop shadow.
Up until this point, I have kept everthing on separate layers just in case I need to make ANY changes.
Highlight the two
COLORED DESIGN LAYERS, Hit
Comnd/Contrl + J to duplicate the two layers, then hit
Comnd/Contrl + E to merge.
Turn off all other layers and group them if you wish.
The merged layer is now ready to be moved to another document.
I save this document.
When your ready to use the corner design, move it into the new document.
Place the corner design in a corner.
On a separate layer below the design, use your
Brush Tool to add some color below the design.
This is not exactly the same as your example, but it gives you a general idea of how it could be done.