Introduction
Andy Warhol, a famous American artist who was a key-figure in the movement known as Pop Art. He lived from 1928 till 1987, the year I was born. He was known for various artpieces including the warhol-paintings of Marilyn Monroe where he painted a stencil of Marilyn in various colors.
That's what we're gonna do now.

Stencils
First of all grab an image and make sure it's a decent size. I'm talking about a wallpaper-size here!
I've chosen a picture of a very high resolution of Kate Beckinsale. I got it of a nice model-stock site. Make sure you got an image that is of some quality.
It's always an option to enlarge your image if, let's say, you got a very small image to work with, but the quality will not always improve and will work against you, so make sure you have a nice one! sxc.hu always has some nice quality high-res stocks!

What you do next is cut out the face. Now luckily I chose a picture that was really easy. I could have taken the Magic Wand-Tool and selected both white area's around the face, but instead I used my trusty Pen-tool.
Some sort of skill is required though, but you can always use the Lasso-Tool and the Polygonal Lasso-Tool!
When you select the head, make sure it's on a new layer. Also make sure you got a layer behind it with a desired color, or leave it empty. We're gonna use that layer in a bit.

Now comes the fun part! Go to Image>Adjustments>Threshold and play with the slider. It's different with every photo because it looks at the light-dark contrast and the use of light in the picture. Make sure you play around with the slider long enough to know what the best option is. If every setting looks bad, you probably need to get a better pictures, Too light pictures or pictures that are too dark are very hard to work with.
You can always press CTRL+L to play with the levels by clicking on the curve and playing with it. Once you have a nice setting, press ok and we can continue...

With the same layer selected, go to Select>Color Range and a pop-up opens like below. You'll notice that once you hover over the image, you'll see an eyedropper. Click on either the white or the black.
What this option does is select the color you click on. In this case, the image has 2 colors. Black and White. And just make sure that, in this case, you click on either the white or the black and press OK.

Bringing Color to the mix
As you can see in the image below, I clicked on white area. Now if you clicked OK, you'll notice a selection has been formed. Open a new layer and fill it with a color. I chose green.

Now CTRL+click the thumbnail of the black and white image and you will select the full image. Then make a layer above it and under the green layer and hide the black and white layer. Then fill this empty layer with dark greyish blue.

Now remember that empty background? Well now we can fill it, or if you already have, you can play with CTRL+U. Just grab the paintbucket again and fill the background with yellow. Or move the sliders when using CTRL+U.

Now this step is vital and you must listen well! We now have 4 layers. A background, a hidden black and white photo, a blue layer and a green layer.
Now you can remove the black and white-layer if you want to. We don't need it. What you need to do now is save this image somewhere in a directory where you can find it. All you need to do now is change the colors of all the 3 layers to different colors and make up cool combinations I went with the classic colorscheme from Mr. Warhol himself. But you can choose any colors you want.

When you're done with all the images you want, open up all the images and grab one of them.
When you're going for what I'm going for (3x3), you'll need to go to Image>Canvas Size and enter the size of your canvas in Percents. Make sure that both options are set to 300. That way, the canvas enlarges itself with 300%. Now that you have your large canvas, drag all the image into that large document and place them next to each other. Once that is settled, go to Image>Image Size.
Instead of the canvas, you're now gonna change the image. Set it to Percents again and choose 50 or 60 percent for both. If that's still too large, you can always make it smaller. If it's too small, press CTRL+Z.

Conclusion
Once you're done, you got yourself a lovely Any Warhol-styled painting! Now print it on canvas and hang it in your home! Remember that you don't need to make 6 versions of the image; You can also chose to make 4 or just one. Or maybe even a whole wall filled with them?