This is mostly for my own personal illumination, and isn't directly related to any school or work projects. I just picked up a trio of laser pointers (red, green, and blue), and I notice that when I project them, the red and the blue appear to be dimmer to my eye than the green one. I had a fleeting suspicion that, perhaps this is an effect of blue and red being at the periphery of the visual light scale, but I honestly have no idea if this is the case or if it's just my eyes playing tricks on me. All three lasers have the same nominal strength, in this case.
It shows clearly how certain wavelenghts, mostly around the yellow-green portion of the spectrum, are absorbed more strongly, and by more types of cells, than the rest.
So it is normal that, even with equal powers, some colors are seen brighter than others. Actually, digital cameras often filter their CCD array with a Bayer mask, which has twice as many pixels filtered green, as red or blue, to better simulate the eye's color sensitivity.
Its because human eyes are more sensitive to green light than other colors. Here's a reference: http://www.physicsclassroom.com/class/light/u12l2b.cfm
I also heard from a friend, although I can't find a reference, that traffic lights display red at a higher intensity so that it appears just as "bright" as green light does.