The responsivity of the human eye is not linear.  In particular it is more sensitive to green wavelengths, so green LEDs with similar intensities to other colours will always appear brighter.

However when designing red, green & blue LED lighting systems remember that these 3 types of LEDs are quite different. You typically electrically need much more red and very little blue to achive the correct mix for a white output. The main reason is that the red LEDs typically have a 2V voltage drop while the green and blue ones have around 3.5V voltage drop and will therefore consume 75 – 80% more power than the red LEDs. Also, blue LED’s typically put out around 50% more radiometric output than the green LEDs.

When talking lumens or candela or MCD, the typical photometric output ratio you need to acheive white is:- 30% red, 60% green, 10% blue.