Learn how to put a gradient overlay on your brushings.

Step 1.
Go to File >> New >> Make your width/height >> Ok.

Step 2.
Click on your brush tool:

Step 3.
Brush whatever you want. This is how mine looks so far:

Step 4.
Make a new layer and set it as Lighten. [We will change the opacity later]

Step 5.
Click your gradient tool and drag one side from another like this:
[DRAG IT ONLY FROM THE LEFT SIDE OF YOUR PICTURE TO THE RIGHT SIDE LIKE THIS:]

Step 6.
Change the opacity to whatever you wish, I chose not to change the opacity, because when I tried, it looked terrible.

Step 7.
Press CTRL + E. Apply File >> Save as... >> Ok. And that's it.