Cobalt isn't blue as other people pointed out.
http://en.wikipedia.org/wiki/CobaltQuote:
Cobalt (ˈkoʊbɒlt/ KOH-bolt or /ˈkoʊbɔːlt/ KOH-bawlt)[3] is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.
Mythril was greenish in XI... ask the Dev why they made it blue in XIV.
Tho, the reason is probably as simple as because one is a fictional metal while the other is a real metal/element found in nature.
(fictional)
http://en.wikipedia.org/wiki/Mithril
(real)
http://en.wikipedia.org/wiki/Cobalt
You're probably confusing the color thing with this http://en.wikipedia.org/wiki/Cobalt_(color)
On the other hand, what about the Mahogany Pattens (Blue) that are white?
http://ffxiv.gamerescape.com/wiki/im...ue%29_Icon.png
While the regular, uncolored, ones are Blue?
http://ffxiv.gamerescape.com/wiki/im..._Mind_Icon.png
