1. We start with a new Photoshop document by press Ctrl+N (or Mac equivalent) and changing the document width to 100 pixels and the height to 20 pixels.
2. Select the Gradient Tool (G), then click on the Gradient Editor to select or create a new gradient.
   
3. Click and drag the Gradient Tool the entire length of the document. You should have something similar to the following image:
   
4. Select Mosaic from the Filter menu:
   
   then set Cell Size to 10 pixels and click OK.
   

And that’s it! You’re finished. You should have an image that looks like this:

You can use the Eyedropper Tool to select your colors or you can repeat the steps, except make your document longer and thinner to create a horizontal bar.

Follow this [link] to download this tutorial in PDF format.