Human perceived (relative) brightness of indicator LEDs
I finally got my hands on some known diodes (as opposed to grab bag stuff) and adjusted them to equal perceived brightness (3 human observers) at a comfortable intensity for viewing from 15 feet (4.5m) away under good office lighting with the desk task light on.

Led part    (Vcc=5.15v)          nm   mCd  Vf@mA    Ohms   mA   Vdrop  E-mCd
WP3A10ID    HE Red (GaAsP/GaP)   627   30  2.0 20    470  1.0    1.84     3
WP132XGD    Green (GaP)          565   30  2.2 20    470  1.0    1.84     3
WP132XYD    Yellow (GaAsP/GaP)   590   30  2.1 20    360  4.3    1.95    13
VAOL-3LSBY1 Blue (InGaN/Saphire) 470  700  3.5 20    33K  0.2    2.47     7


nm, mCd, and Vf@mA are from the data sheet.

The last columns are the resistor I used, the calculated current, the voltage drop across the diode, and the calculated effective mCd at my reduced current.  The supply voltage is 5.15V, and all diodes are T1 format with diffused 60 degree viewing angle lenses.   

The blue diode was the lowest intensity (mCd) which I could find, and you can see I had to scale its current way back.  All LEDs are well under-powered from data sheet values.