Light meters such as used in photography have varying sensitivity to different wavelengths of light. That is because they are based on small PV cells and PV silicon has varying sensitivity by wavelength.
It doesn't matter for most photography since the light source is sun or flash or light bulb which are emitting all of the visible spectrum as well as non-visible spectrum too (e.g. infrared and ultraviolet), and whatever is reducing the light level (shadows, usually) is reducing all wavelengths of light more or less equally - "more or less" being in the very crude context of photography where you're only trying to get within an f-stop or so (1 f-stop more = 2X the light level), not exactly high precision.
The human eye also has varying sensitivity to different wavelengths and it even varies between humans.
So, as Aurel says, it's probably not so easy to measure this "precisely". But so what, this is just a kid's experiment. Define "light" as "sunlight at high noon cloudless sky at my house this month", and "brightness" as "brightness as perceived by my eyes", and experiment away!
wavelengths of different colors of light
What Wavelength Goes With a Color?
spectral sensitivity of a (typical? dunno) light meter (scroll to #6)
http://www.dataloggerstore.com/crm_uploads/1336.pdf
stuff on Wratten color filters
http://www.olympusmicro.com/primer/lightandcolor/filter.html
If and when you want to get more fancy w/ light experiments, you can probably rig up cool stuff using photographic equipment. Any old-time used camera store or flea/junk store will yield a $5 camera lens that has a manual aperture and actuating pin, you can use that to reduce the amount of light that is transmitted through the lens in a more or less calibrated way. A slit in cardboard, and a prism bought cheap from an education supply store will separate that beam of white light into colors. A good photography store will also have color filters (sometimes called Wratten filters) that are calibrated to transmit X % of light of a specific wavelength, glass ones cost a bit but gel ones are pretty cheap. Fun stuff like that.