[Photoshop] Fill selection with random colors

You can use the scripted fill with a script that I wrote, which is available Here: Scripted Fill UI - Photoshop CS6

By creating different colored patters like this:

You can use my script to change the selection of the pattern to each of the different patters. There are setting you have to do do make them line up:

Use TempFillScript as the script. Once you click the okay button on my UI, the standard UI will show up the number of times that you selected for different patterns.

You could try Phiotoshop's Symmetry Fill, which has a Color Randomization feature, but I find it fiddly and hard to get the results I want.

While a script would be a first choice an alternative would be to use your sample as a segment in an overall design by flipping the existing file vertically and/or horizontally and butting or overlapping them in register. It would give the illusion of random placement of the filled-in areas. Since your current design is not precise, it would have to be reconstructed. As you can see from the example below, register is not possible. With accuracy in the first segment this arrangement can be repeated endlessly in both width and height.  (Check into Edit > Transform's Step-and-Repeat feature.)

(Color lighter so that the random pattern and misregister are easier to see.)

You may be able to script something like that. The script would set random magic wand selection contiguous low tolerance to select a current color then fill the selection with one of your three colors. However, there a good chance you will sometime select the dividing line color so you should test the bounds of all selection and not fill selection that select the line color.