The metal that best fits these properties is Aluminum or some Aluminum alloy, the resistance to rust, density, color and hardness all match for untempered aluminum. The tempered form would have to be Anodized aluminum which is a coating of Corundum (hardness 9!) which is created by immersing aluminum in acid and running a current through it, a sophisticated process but one that mechanically inclined dwarves could conceivably do with a primitive chemical battery. The process would be particularly effective on something like ring mail due to the high surface to volume ratio.
No, aluminum is a horrible choice. The regular metal may be about as soft as copper, but it fatigues far quicker.
Corundum, also known as
sapphire, is as brittle as glass, and a coating would crack the instant the material is dented or bent. It's also only
equal in actual
strength to beaten copper (about 300 MPa tensile). Human hair has a tensile strength of
380 MPa, making it actually stronger than copper or corundum. Do you like me new armor? It protects me worse than me beard!
Titanium, on the other hand, isn't a
bad choice for mithril, but not great. Pure titanium is about as strong and soft as copper. The absolute best titanium alloys are about as strong as steel when both are at middling hardness, but the hardenability is complete poop. A typical high carbon steel can be hardened anywhere from 180 Brinell to 1800 Brinell (though anything over about 750 is very brittle). The strongest titanium alloys harden from 300 Brinell up to 370. 370 is a good hardness for armor, though. But swords work best around 500, and knives should be around 550-600.
Really, best bet is for mithril to be some magical junk, instead of a real chemical element.