Brushes are great, but they only work with a single colour. If you want to add multiple colours or gradients to your brushes here are a few ways to do it.
Gradient Overlay method
First make a new layer, and place your brush design on it – the colour doesn’t matter. I used brush number 2 from my Gradient Curves set.
Go to layer > Layer Style > Gradient overlay. Choose the gradient you want or make a new one, using the settings below. make sure preview is on so that you can experiment with the setting until you’re happy with the result.
And here’s how it looks. One brush, three blended colours..
Feathered Selection method
For more complex brushes, you can use the selection tool. Again start by putting the brush on its own new layer. I used a brush from my free Swirls and Seeds set
Using the freehand Lasso Tool, select a portion of the design you’d like to change. Go to Select > Feather and choose a number from around 20 to 100 pixels, depending on the size of your design. Feathering the selection means you get nice soft blended gradations of colour – the bigger the feather, the softer and more gradual the blend.
Go to Image > Adjustments > Hue/Saturation. With the preview on, move the hue slider until you get the colour you want. You can adjust the lightness and saturation too.
If you repeat the process on different areas, you’ll end up with something like this.
Magic Wand method
Lastly and most simply, if your brush is a clean edged one you can just select all or parts of it with the magic wand tool and colour within the selection. You can use pattern fills too as shown below. I used a design from my free Cells set to create this design from just one brush. I also used a pattern to fill other sections. (You may have to flatten the image before it will allow you to select parts of the brush).