Here is a great way to make seamless textures.
http://neotextureedit.sourceforge.net/
I made a simple 3 color retro camouflage starter.
[attachment=2:12198s4e]camo.zip[/attachment:12198s4e]
Just open it from inside Neo texture edit. It makes old camo like this:
[attachment=1:12198s4e]kamo.png[/attachment:12198s4e]
Here is an example of the settings needed. Just look at the red arrow. There are 3 bars in the center that are maxed out. The rest are turned off.
[attachment=0:12198s4e]Settings.png[/attachment:12198s4e]
That is it!
You can change the colors by adjusting the color swatches, on the gradient editor. I am going to be making some desert and some Europe WWII color schemes later.