From a bug report two years ago, mithril armor should have color
HI_SILVER rather than HI_METAL. Tolkien describes mithril as "like
silver which never tarnishes". (The bug report had a more precise
quote, and a github URL for a patch which I've ignored.) The Amulet
of Yendor has material set to MITHRIL but I haven't changed its color.
All amulets are HI_METAL, so if any modification is needed for it,
change the material rather than the color.
Mithril-coats are displayed as cyan both before and after this patch,
so it's hard to tell whether any change actually happened.
cd9e67a12a