To screen print an image with more than one color you will need to make a separate stencil for each color. This means you will need to print out transparencies to expose with, and you need a program that will easily allow you to prepare the images for printing. A good one for this is a program called GIMP (GNU Image Manipulation Program) because it is free and has a lot of powerful features. Doing color separation is very easy with GIMP and will not take much time at all. Begin by opening the image you want to screen print. You can drag and drop into gimp or go to File>Open and chose a file. The last thing to do before we print out the transparencies is change all the images to black. Chose the color with Select by Color tool, then go to Colors>Colorfy. In this menu we can chose the size we want the image to be by inches or pixels, ect. I want this image to take up a whole page so I am setting the size to 8 by 10.666. Once I type in 8 fior the width and hit Enter it will automatically set the height. I did trial and error with the width untill i knew it would print on a 8.5 x 11 transparency.
Be sure you are using an image with a high resolution. If you try to scale your image and it begins to look pixelated, your image is too small. You may try finding a larger image or just print the image smaller. Now we want to control how large the image will be when you print it. It is important we do this now before making any other changes to the drawing because we want the the colors to print at the same scale. This way our stencils will line up perfectly when we are done.
Go to Image>Scale Image. While Select by Color Tool is selected put the cursor over a area and click it. I have chosen to start with black.
If you have trouble grabbing the entire color area use the Threshold bar on the Toolbox. Be careful, the higher you set it, the more likely it will select colors that are similar to each other but not identical. Now we want to copy and remove the black area. Use the Cut tool or hit Ctrl+X. Now we need a place to put the black layer in for printing. Go to New. Now the color you selected should be completely gone. If it is not, undo what you did and try setting the threshold higher. It will open a box for creating a new image. You will see it automatically matches the size to what we already set it to: 8 x 10.666. This is what we want. Now we have a second blank page that is the same size as the original. This is very important, we want all the images to print at the same scale so they will line up for screen printing. Now paste the layer you just removed into the blank file.
You can keep creating files and pasting the colors you remove from the original for as many colors you are screen printing.
If you are printing a 4 color design, you will need 4 images for transparencies. You can see it placed the black design in exactly the same size it was in the original. If the image goes out side the border when you paste it, or is noticeable smaller than it should be, undo the paste and go to Image>Scale Image and make sure the size is identical to the settings in the first image. Then repaste. Now we want to control how large the image will be when you print it. It is important we do this now before making any other changes to the drawing because we want the the colors to print at the same scale. This way our stencils will line up perfectly when we are done.
Go to Image>Scale Image. In the Colorfy box go to custom color and chose black. It will give you a preview of what it will look like. Hit OK. Now they are ready to print.