I would love to see your statements, especially crutschow's, laid out in in an example.
I have two LED chains (4 LEDs in series), one chain with Vf of 3.2V and I=30 mA for each LED.
The other chain with Vf of 2.5V at I= 20 mA.
Both chains are connected to the same 12V supply and make up one small circuit board.
Currently I connect one chain directly to the 12 V supply, figuring 4 X 3.2V, that is roughly ok.
The other chain is connected to the 12V thru a 100 Ohm limiting resistor.
Given your reasoning I should increase the power supply voltage, adjust the 100 Ohm resistor and add another limiting resistor for the 3.2V @ 30 mA LEDs!?!
A constant source power supply would not work in my situation, I think, since I use many of these boards, switched on and off rather randomly, so the load varies widely.
And the power source has to accommodate this widely changing current.