Henrik;
Period photos show that both planes and tanks could appear wearing "soft-edge" camo schemes. The paint spray-gun was invented well before WWI - the earliest forms were patented perhaps as early as 1880!
Spray-guns were in use by the Germans and captured Mk IV could well have been painted with such. I have seen several period pics that indicate sprayed patterns.
Google "captured British Mk IV tanks in WWI". Check out the "images" tab...
An interesting pic of a Mk IV showing clear sprayed camo shows up at:
http://upload.wikimedia.org/wikipedia/commons/5/5e/CapturedMkIVFemaleTankInGermanService1918.jpg Now... as to the actual colors being used... My best suggestion is to search for the colors used by the Germans to paint camo on artillery pieces in WWI. That, and maybe some other ground equipment.
I would be thinking that you might use the colors which later appeared in the German pre-WWII "Feuersicherlich Buntfarben-anstrict" camo-scheme used by the Heer ca 1934 - 38. There are some "on the web" sources that identify German equipment colors from WWI through WWII. Check those out!
Hope this helps!
Cheers!
Bob