You can use green led as current source , the only difference is 2.2v Vf instead of of 1.65-1.7V for red LED. A 150R Re for the red led might be a 180-200Re for the "greenie" , no biggie!!😀 Do the math . (or let the sim do it for you)
I use all 4 colors (blue/green/red/yellow) , even for CCS's. 🙂
OS
Have you done temp coefficient experiments - with GREEN vs. RED?
I did this some 15 years ago.
Easy to do.
You attach the Transistor and LED close together.
Use 2 pairs, T+RED and T+GREEN.
Use a suitable resistor for each pair.
Attach 2 multimeters. Monitor voltage across each package.
Take a lamp, 60 Watt. Switch on and warm up the LEDs/Transistors.
Take measurements and write it down.
Now you can calculate % change of voltage across both resistors at certain temperature change.
Regarding temperature we have first room temperature.
Then we have the heat generated inside the amplifier closure.
Changes of some 10-20 degrees Celsius is not improbable.
And the main reason for RED, is it follows the transistor VBE better as temperature changes.
How big deviation increase with GREEN, will vary, because there are plenty of different LEDs.
But, ostripper, believe me,
it is not by chance RED LED is used as CCS by many serious amplifier designers.
Of course using 2 transistors instead, also is great.
Like you sometimes recommend.
