I have a habit of sticking citrus seeds in the soil of a handy overwintering plant, then separating them out when and if they grow. For a while I didn't bother to label the seeds so I have a bunch of seed grown little trees that are unidentified.
Last year, I poured over online citrus identification pages and tentatively ID'd this one as a lemon or a Mandarin orange. Today, I was wondering if the bronze new leaves have any significance, and it seems that lemons are more likely to have bronze new foliage.
... Ignore the light colored foliage in the background -- that's another citrus that didn't take kindly to being left outside as long as it was. (I'm assuming its a hint that that one is a more cold sensitive citrus compared to the "lemon")