Here is a quick and easy guide to make good camo:
Step 1:
Open a new file any size you want, during this tutorial ill be using a 500x500 square.
2. Make your background colour #005E20 and your forground colour #8DC63F
3. go to filter>render>clouds

4. go to filter>render>difference clouds, repeat this step until you get a result that looks like this:

5. Go to filter>artistic>cutout and set the setting to this:
no. of levels: 8
edge simplicity: 0
Edge fidelity: 0
and you should get something like this:

6. Go to filter>noise>median and set Radius to 3
and you should get this:

7. go to filter>artistic>dry brush and set to these settings:
Brish size: 2
Brish detail: 8
Texture: 1
you should get this:

Look what happens when you overlay it onto objects, such as weapon skins for counter strike source!

Enjoy, i will update this tutorial as i make it more like camo

Xhiea
p.s. the m4a1 skin is avalible here (you need winRAR to open this file) ----> CAMO M4A1 SKIN