In the days of film photography solarization resulted from extreme overexposure. A pseudo-solarization can be created in the darkroom by taking the picture normally and then during the development phase exposing the image for a few moments to white light.
With photoshop we can create similar solarized images. The process is to convert a normal color image to black and white and adjust the image so that there is reasonable contrast. In Photoshop the process is Filter/Stylize/Solarize and an image similar to a film produced solarized image will result.
In addition to color solarised images, images can also be converted to monochrome and then solarised. An example is shown below.