This might be a long post so please forgive me I will also try to post picture examples. I'll try to keep them small though.
Here is a very simple and easy filter you can use in Photoshop and most likely Paint shop Pro to make camo. However, I am using Photoshop so I don't know the filter names with PSP as I've never used it. As Ruin mentioned the VDL camo plugin is very easy. I do recomend it for basic camo.
This may not work well with pure black or dark colors. Make a new document and set the pixels to whatever size you'd like. Pick a color. I used an off green color hex code 414D44 Fill the layer with that color
Now, go to Filter, Artisic, Sponge. Now you should have 3 options. Brush size, Definition, and Smoothness. If you set all of them to the max (all the way to the right) you should get something that looks like this
There you go. Instant Camo!! It's that easy. Experiment with the settings. Brush size is exactly what it sounds like. It's the size of the brush. Definition makes the image clear or blury. Smoothness will smooth out the brush edges. If you'd like to make a really easy cheap Digital Camo. Simply set the brush size to whatever you like. Max out the Definition and set the smoothness to 1. You'll get an image that looks like this.
There you go. An quick easy way to make a camo