Even tho I have not attempted to make any skin's yet these are 2 books I have picked up for reference's first book is by Rosch and it list all the color codes and markings for everything related to German aircraft from 1939 to 1945. I am still reading thru the page's and it almost looks like a Part list book for color codes.
All the photo's they give in this book are done in black in white
Another book I have to give me a color reference is this one below for when I am looking to see what it would look like
Once again I not attempted to make a skin yet to many other projects in the way and I am not sure if these books would be good reference's but they to me I think would be a good start along with all the other recommendation's given so far by seasoned skinner's here in the game.
Keep on trying you only get better by mistakes made.