Recently I realized that green LEDs are much brighter compared to red LEDs. For example for SML-LX0404SIUPGUSB RGB LED the luminous intensity is 30, 40, 20 mcd at 2mA for red, green, blue respectively.  

Then I grabbed a red LED, which I thought was pretty bright (250mcd@20mA), because I selected it for my previous blinky circuit, and soldered it in series with a green LED that I recently selected (1400mcd@20mA), and guess what? The green LED was so much brighter. 

And look at this, at 40uA, it is still pretty bright. Even though the forward voltage is a bit higher, about 2.2V vs. 1.7V for red, it is still much more efficient and the reason is that the human eye is most sensitive to green.  

Then I ordered a bunch very bright green LEDs from Wurth

I set the current only 10uA.

Compared to a smartphone these LEDs at 10uA are very visible. I really need to design a solar powered blinky with green LEDs.