The mcd spec's don't account for the response of your eyes.
actually, the
candela or millicandela does account for human eye sensitivity ... same goes for the lumen, although the measurement of lumens and candela are different, they're both based on human eye sensitivity, or "visible light". so, if you find three diodes, red, green and blue, with nearly equal mcd ratings
and viewing angles, they will appear equal in brightness to the naked eye
there are formula that help you mix led output based on the CIE chart, but it's a whole lot of math and you have to rely on spectral emissions data from the manuf, which may or may not be accurate for the exact LED you have.
depending on your application, you could use PWM to set the levels of the led, reflect it off something white, and adjust the levels until you get a satisfactory color.
if you want a good bright "cool" white, you'll want two blue emitters for every red + green pair. if you want a good bright "warm / neutral" white, you'll want two green emitters for every red + blue pair ... LEDs of these type normally come in the 10mm (round) package, the piranha (superflux) package and the barracuda (luxeon) package.
if you have access to a technical library or a university nearby, these publications might interest you:
CIE 013.3-1995 Method of measuring and specifying colour rendering properties of light sources
CIE x013-1997 Proceedings of the CIE LED Symposium '97 on Standard Methods for Specifying and Measuring LED Characteristics
CIE 127-1997 Measurement of LEDs
CIE x022:2001 Proceedings of the 2nd CIE Expert Symposium on LED Measurement "Standard methods for specifying and measuring LED and LED cluster characteristics"
CIE 167:2005 Recommended practice for tabulating spectral data for use in colour