Always nice to learn something new.Where different types of walking route overlap. White-Red is a GR route, White-Red-Yellow means a GR and a PR route follow the same trail. And White-Red-Yellow-Green mean that a GR, PR and SL route all follow the same physical path.
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GR / PR / SL Walking trails
European walking trail network GR, PR, SL walking trails in Spainwww.bridgetospain.com