It's one of those systems that doesn't quite work the way the devs think it does, probably from having the dev teams having about 500% turnover since AoS.
Since at least Mondain's Legacy, the devs think that humans have a base x% of getting colored ore, and elves get a 20% increase (while humans have a 10% of an extra ore).
In reality, the chance of colored ore varies by resource square, probably ranging from about 10% to 80%, and stays the same regardless of what the current ore type is (if a site is currently iron only, Prospector Tools and Gargoyle Pickaxes can elevate it to color, and the colored chance comes back into play). Elves get a 20% boost, but in terms of the base chance times 20%. For example, if a human is getting 60% colored ore, the elf will get (60% * 1.2) = 72% colored ore.
Similarly, the number of digs you get from a spot remains roughly the same, only the colors may change.
Many people see a marked performance increase with an elf miner, but it just doesn't feel right to me (given the whole elf = nature thing). I tend to stay with a human miner for RP and load-carrying concerns, and since I mine almost exclusively with gargoyle pickaxes (and using a prospector's tool with it), I get enough ore elementals to compensate for the lower mined ingot totals (plus, it's not as boring).