Green indicator LEDs in stock at Digi-Key with status of active, with performance specified at 2 to 30 mA, and wavelength indicating InGaN chemistry which favors very high efficiency at low current. These should have intensity of at least 6 mcd at 1 mA and at least 15 mcd at 2 mA. One item has typical forward voltage drop noted as 2.1 volts at 2 mA, I believe it is 2.5 to 2.75 volts at 2 mA.
Blue indicator LEDs in stock at Digi-Key with status of active, with performance specified at 2-30 mA. Ones with voltage rating less than 3.5 volts have InGaN chemistry which favors very high efficiency at low current. Such ones rated 15 mcd or more at 20 mA or less should have intensity of at least 2.2 mcd at 1 mA and at least 5.5 mcd at 2 mA.
Indicator LEDs in stock at Digi-Key with status of active, with performance specified at .5 to 2 mA.
The above links to Dig-Key were corrected on 6/12/2022 and are still working as of 9/9/2022. If these fail to work, or if for any reason you want to do your own work at finding at Digi-Key LEDs that are useful at low current, then Go Here.
Update 7/26/2020: "Straw hat" LEDs generally have a wide viewing angle of 80-110 degrees and InGaN green ones are typically useful at low currents of .5 milliamp or less, even if they are from cheap suppliers. Update 9/6/2022: These are often referred to as 4.8mm LEDs.
Good reasons to use such LEDs, as well as additional specific part numbers including yellow ones.
Link to pulsing circuits for extreme LED efficiency at extreme low average currents. Most LEDs have best efficiency when instantaneous current is 1-10 mA, and pulsing circuits achieve optimization of efficiency when average current is .04 - 1 mA. Also mentioned is one LED with efficiency so good at extreme low current that pulsing may not have much benefit.
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