Both of these flags reuse the colours of the national ensign.
Upon their heraldic shields are matching elements. Both have an English St. Georges Cross look a like. The red cross on a white is a very popular symbol. Officially it represents the Archbishopric of Trier. The crowned Gold Lion on a black background also represent the same herald - the Elector of Palatinate.
Saarland and Rhineland-Palatinate are in the twilight zone in between France and Germany. Essentially the Rhine River that starts in Switzerland and ends in the Netherlands is somewhat of a natural divide between Germany and France. But the Rhine has a German tilt, likewise Switzerland is more 'German' than French, just as the Dutch are more closely related to the Germans. Germany was called "Deutchland" after all. In Pennsylvania the great misnomer of the Pennsylvania Dutch has confused common folk into thinking that they are Dutch when they are really in fact German.
After WWII Saarland was plucked away from Germany and ruled by France from 1947 unto 1956. In a way Saarland is the Okinawa of Japan, because the US plucked Okinawa away from Japan and made it US territory. But since Okinawa was mostly Okinawan and the Japanese were being a loyal allies, the US returned it in 1972. Likewise France allowed Saarland to return to Germany a wee bit earlier. Finally just as Saarland is middle point between Germany and France, Okinawa too is a middle point in between China and Japan. But both Saarland and Okinawa have German and Japanese tilts, respectively.
Also instead of beer being their preferred choice of 'fungal dope' the people of Saarland and Rhineland-Palatinate are more like their French-siblings to the West with a fond love growing and dining with fine wine.
Link to Part IV