Good morning Fabricio!The line infantry(not the guard units) and artillery regiments have red cloth numbers in peacetime.After mobilisation they must take red waterproof clothcolour or green after 15.08.1914."R" for reserveregiments,"L" for landwehrtroops and a iron cross for Landsturmunits with numbers under the characters.At the end of 1914 they must give this system up.Some units turn their covers about 180°.A good source for German WW I-uniforms is :Jürgen Kraus,Die feldgraue Uniformierung des deutschen Heeres 1907-18.ISBN 3-7648-2533-2.
Cheers,Thomas.