Worst Case Analysis of the LED circuit - Reg
And what about the missing temperature vs change in voltage graph of the LED.


Your link for the first datasheet did not work for me.  

The graph you refer to is a graphical representation of the LED's  Vf temperature coefficient. 

From the green datasheet the  Vf temperature coefficient is 0.0036 V per °C (°K).
The yellow datasheet should have the Vf temperature coefficient even if there is no graph.  



While luminous intensity may not be relevant to WCA it should be noted that temperature has a huge effect on the luminous intensity of a yellow LED.   

  

For WCA it is better to use the temperature coefficient rather than the graph. 

The green datasheet states the Vf values (2.6V, 3.0V, 3.7) are at 25°C. 

That should mean (for the green LED) at a case temperature of-40°C  will add (negative coefficient) to the VfVf25° + (65° x 0.0036V) and at 80°C subtract (55° x 0.0036V) to the min, typ, and max values.  

Regarding @SunnyskyguyEE75 answer, I would not be concerned with the Tja for this circuit. 



Tja is not very relevant to an  20 mA circuit.  

Even if using a high power LED, you cannot use the specified Tja as this spec is for a JEDEC test board for comparison purposes and is unlikely to represent your circuit board.  


  The junction-to-ambient thermal resistance, RθJA, is the
  most commonly reported thermal metric and is the most often misused.
  Source:  Semiconductor and IC Package Thermal Metrics




If you were to use Tja you would need to understand the thermal complexities of PCB design.  IF you have an interest in PCB thermal management this is a good reference:  Thermal Design By Insight, Not Hindsight