This isn't really a tutorial but more like a reference guide. Feel free to add any other information to this if you'd like.
*Please note that I don't claim any expertise in this subject, most of this falls under the heading "generality" and may have holes and errors.

I'm not going to get into the details of what planes to put the camouflage on or which is used where or is historically accurate. Just know that you'll need to dig into some HEAVY research on the plane you want to paint if it will be historically accurate. If your intention isn't historical accuracy, please have fun as possible!
Here is a short description-
In April of 1917 an order was given that all German planes would be covered in preprinted camouflage fabric. Of course there are many exceptions.
Why the funny shapes and colors? No one knows for sure but many plausible theories exist. Suffice it to say that planes were generally covered in printed camouflage fabric of multiple colors.
The standard day camouflage was in the form of irregular polygons in darker shades for upper surfaces and a lighter shades for the lower surfaces. Basically 2 sets were used, 4 color and 5 color. There was also a marine pattern, only for upper surfaces, using regular hexagons.
Also a night camouflage pattern existed.
Austria-Hungary had their own camouflage not to be confused with the German type.
I'll concentrate on the 4 and 5 color camouflage as I've done no real research into the marine and night patterns.
This is the complicated part, no one can agree on what the exact colors are! :P
There are many original fabric samples but they are not all the same. That is to say the colors. The pattern itself never varies, or enough to make a difference. The 4 color and five color fabrics are very uniform in pattern. But color...OH...

There are theories and data to explain it partly, then there is the time factor. Has the color on this fabric changed due to age?
On to the 4 and 5 color fabric-
I've come up with a number of various color combinations using what I've got in books and also several great online sources.
My samples-
(Mark Miller)
(Aerodrome Forum)
A zip file of a PSD with 21 layers, labeled as to who's color research was used on each is attached. Fun to play with. (if you like this sort of thing)
*greatly reduced in size*
new_lozenge.psd.zip