The colour of the signed is blue, not yellow.
Yellow is the colour of the local hiking trail (PR : petite randonnée).
Or yes, this is the colour of the MTB symbol (it's mean local route).

Blue, in practice a MTB logo with the number of the route in a blue square.
It mean easy.

See https://sitesvtt.ffc.fr/le-balisage-du-parcours-vtt/

Best regards,
Yves — Groupe OpenStreetMap Plein Air