To investigate this, use the yellow, green and blue LED and measure the forward voltage.
You will be surprised how great the differences are. Typical values for the forward voltage are...
Your actual measured values may differ greatly from this. The forward voltage drop is determined by the used semiconductor material and this information you find only from data sheets.
|infrared||< 1.9 V||GaAs, AlGaAs|
|red||1,6 - 2.0 V||AlGaAs, GaAsP, AlGaInP, GaP|
|yellow||2.1 - 2.2 V||GaAsP, AlGaInP, GaP|
|green||1.9 - 4.0 V||InGaN, GaN, GaP, AlGaInP, AlGaP|
|blue||2.5 - 3.7 V||ZnSe, InGaN, SiC|