Can anyone please suggest the brush i should use? I have tried using gradients and various brushes with noise (without noise)...just unable to get the sharp edges and the mix of the colors.
You can make a similar effect using color elements in different layers and filters, no brushes:
Add a Gaussian Blur Filter to each layer:
This is the result:
If you need a coarser blend, duplicate the layer, send it back and change the blend mode to dissolve. In this example I added a Maximum Filter to make each back layer smaller:
To make a complete simulation of Ana Montiel's work, use color fill layers and masks:
The grainy brush effect can be made with blur and noise in the layer mask:
In the left there's a circle (=filled circular selection) as a layer. White is the background layer. The same selection was ON when the layer mask was inserted, so the mask covers exactly the circle.
In the middle a copy was made, moved aside and blurred with Gaussian Blur. The mask got the same blur without asking.
In the right the mask was selected in the layers panel and about 15% noise was inserted (=Filter > Noise > Add Noise)
Applying 1...2 px Gaussian blur to the noisified layer mask the result is different. Its tried to a blue copy:
In the example there was the problematic partially sharp shape. One can try to combine blurred version of a shape with a non-blurred version with layer masks or erasing parts of both. With very careful work one can get somehow smooth transition between blurred and non-blurred version, but somewhere both versions are visible. I guess that's not wanted. The amount of blur should increase gradually.
In Photoshop there's the radial zoom blur. It can easily make the wanted effect. Make a big selection which has the center point in the middle of the wanted sharp looking area:
Apply Filter > Blur > Radial > Zoom. It blurs both the layer and the layer mask. You can apply the blur twice if the effect was too light. The effect is shown in the layer mask. You can get the mask onscreen by clicking the mask icon and holding the Alt key at the same time.
The total result after adding some grain to the mask and inserting one colored shape more to the image:
There's also other ways to make place dependent blur:
1) Lens blur. Simulated field of depth can leave a sharp area and gradually increase blur around it. It's a little complex in Photoshop and extremely slow, many other photo editors or Photoshop plugins have made it easier. In Photoshop you must make a layer mask where non-blurred areas are black and there's gradual change to white where you want the transition to blurred occur. You must paint white where you want the blur enter to. After applying lens blur you disable the layer mask.
2) Manual blur tool; you select how much you rub with it.
3) Painting manually; the sharp part with selection and the rest with feathered selection or without selection with smooth low opacity brush
Applying Gaussian Blur to a feathered selection doesn't do the job. Blurred and non-blurred versions will be both visible in the transition zone.