Do the colours of the Club Vosgien waymarkings reflect the difficulty of the trails?

No, the colour of the markings is unconnected with the difficulty of the trails. You can find more information on the Club Vosgien website.

The markings change regularly along the Park’s 6,300 km of waymarked trails. Have a map ready before you set out, and study your route so that you can make your hike unencumbered.