The only original area that was white on a luger was the Gieschert (sp) on the left side for the safety.
All other whiting was added at some point by a collector or someone who wanted it to stand out.
"Usually" it is grease pencil or the like and hoppes gun oil or other nonintrusive oil will take it off using a toothbrush.
Welcome to the forum, read around here, read the FAQ, see what other 1917 Erfurts are supposed to have for matching numbers etc.
The old adage is buy books, then guns. Many folks don't follow that advice
Ed