The LED will have to be a 5mm T1 3/4 size to fit in a panel mount LED holder.
I went to Digikey and looked through the available LEDs. I found 5 differerent ones that may work, and ordered a couple of each:
(I apologize for using a screen shot of a spreadsheet here, but I don't feel like messing around with table formatting right now.)
After receiving the Digikey order, I put the LEDs on a breadboard side by side to compare them. There are two of each LED, one green and one other color. Here is what they look like:
LEDs from the front in dim light.
LEDs from the side in dim light.
LEDs in bright light (shaded daylight.)
LEDs in direct sunlight.
My observations about the LEDs:
- Amber, orange, or yellow are all too close to green. The indicator LED will have to be a red/green LED.
- The diffuse LEDs look better than the clear one (though the clear one here is also much dimmer than the others.)
- LED #3 is the best of this bunch. It's also the brightest one.
- The LEDs are very hard to see in direct sunlight. LED #3 in red mode is visible, but you have to be looking for it to notice it.
There is one LED from Digikey that looks like a good candidate; I wish I had included one in the order. Digikey P/N 754-1235-ND has specifications similar to the #3 LED, except that the red component is twice as bright. It's also a bit cheaper.