You will find that the prototypes now actually represent groups of pixels that are predominate in the image. Now, you can create a simpler image only made up of y colours by classifying each pixel according to the prototype it is closest too. Here is our picture in greyscale and another that has been fed through the prototyping algorithm built into ED256 . We use greyscale to make prototyping a lot simpler. For another example, look at this picture of a F-22 Raptor.
Dark blue is for horizontal edges, purple for the dark aircraft body and black for the land.
It all boils down to simplifying the data that the computer has to deal with. Less data, the more time can be spent extrapolating features. The trade-off is between data size and retaining the features within the image. For example, with the prototyping example, we would have no trouble spotting the buildings in the picture, but the tree and the car are a lot harder to differentiate. The same applies with a computer.