Start off by opening any photo into Photoshop. This effect works best with isolated objects on black backgrounds such as the image below.
To begin, we'll desaturate the image. This step isn't necessary to get the effect, but it will be helpful when we're painting. In the Layers palette, click on the button and choose Hue/Saturation.
In the Hue/Saturation tool, set the Saturation to -100 then click OK.
Now we'll create a new layer. This is the layer where we will be painting the colors on. Click on the button in the Layers palette. This will create a new layer called "Layer 1".
Change the blending mode of Layer 1 to Color.
The primary colors that we'll be using are yellow, magenta, and cyan. Before we start painting, we'll use the Swatches palette so that we can quickly select those three colors. In the Swatches palette (Window > Swatches), click on the icon then choose one of the PANTONE CMYK swatches.
When it asks if you want to replace or append the swatch, choose replace. Your swatch palette will now have yellow, magenta, and cyan as the first three colors. Now you can select one of these three colors quickly.
Select the tool from the toolbar. Right mouse click on the image and the brush options will appear. Set the hardness setting to 0%. Don't worry about the master diameter setting yet.
Begin by selecting either yellow, magenta, or cyan from the Swatches palette and painting anywhere in the image with a large brush size. You can adjust the brush size by pressing the [ or ] key on your keyboard.
Select a different color and paint the rest of the image.
Now that you're done, you can delete the Hue/Saturation adjustment layer.