"Toning" is the preferred nomenclature. I no longer have this one since someone offered me way more than it was worth for it, but it was pretty orange.
What the devil is Orange? Don't believe I've any of that sort in my collection......however, kudos to those that do.........
It's a Japanese nibu-kin (2-bu) coin from the end of the Samurai period and the beginning of their transition to a more industrialized society. The coin was struck in the Meiji era, circa 1868-69, and is made from an electrum (gold/silver) alloy. Though there's more silver in it than gold, it does have the look of a more pure gold coin. This example is a PCGS AU55 and has nicer toning than the PCGS XF45 I owned previously. These are relatively common and affordable as 19th century gold coins go, since they aren't particularly big (and are only about 1/3 gold, technically). This piece cost me $150, already in the PCGS slab, right here on CoinTalk from toned gold guru @jwitten. The previous example I had I bought raw for around $80, and sent it to PCGS myself.