So in a new document create a shape somewhat irregular (this can be done with a filter {Ripple for example} on a shape) then duplicate it for the other two colors, give each shape a color and create a pattern.
You may need to duplicate the above a few times before creating the pattern so that it fills your backpack. Now you can use the pattern layer styles. Using the pattern layer style allows you to scale and move the pattern into place.
You may want to create selections on your backpack and copy to a new layer so that each area can have its own layer style.
Thats about as far as I can go with out actually doing the job for you. I hope this helps.
BTW, if you have illustrator, you may have an easier time creating patterns.