Silvally is clearly linked to Silver in all its language names and silver is prominent in its design.

What does this mean? Note also how Arceus' ring is gold coloured. If you take Arceus to be gold and Silvally to not-quite-be Arceus, that would make it silver.

Its design also seems to take some inspiration from this alchemical symbol for silver: see its front legs, tail and mane. I admit this part might just be a coincidence.