I don't know how it happened and wish I took a picture of it but Ban-Lu can be a different colour. I was at the NL summoning stone last night when I noticed Ban-Lu was light blue instead of green. Sadly Ban-Lu looks better in that light blue shade. So keep an eye on your companions colours grandmasters (he is the only mount I've used since day 1 availability and this is the first time I've noticed it so it could be a rare bug).
Lol are you sure it wasn't the lighting of the zone? It's common for mounts to look different shades of colors when their color mixes with the lighting in a zone. Heavenly Cloud serpents are a big example.