Calculate number of photons of a light source given the lumen and the color spectrum
The energy of a photon of light is given by
$$E = h * \nu$$  where \$h\$ is Planck's constant, and \$\nu\$ the frequency. So it's easy to derive this:  

\$Photons/s = \dfrac{P\lambda}{hc} \$ where \$P\$ is the power in watts and \$c\$ is the speed of light.

For example, a laser pointer might emit 2 mW of light, at the wavelength 630 nm. The best way to solve these equations is with google, then you don't have to worry about the units.
Google "2 mw * 630 nm /( (planck's constant) * (speed of light))" and it'll say:

Photons are really small!

Now you need to figure out the power density in your frequency of interest. Of course, at a single frequency, the power is zero, so you're looking for the power in a range of frequencies.

Here is the spectrum of a lamp from wikipedia. Lets try to find the power density of the "green light"  



This is in arbitrary units, but no matter, if you integrate it you get the total power of the lamp. My digit-oral-integrator says the total power under the curve is about 6000 nm.units. The integral of the "green light" between 510 and 550 nm is about 800 nm.units, or 13% of the power.

Now all that's left to do is find the total light power of the lamp in watts.
Unfortunately the lumen is not a unit of power; it is, sort-of, but it's weighted by the sensitivity of the eye. For a metal halide light as shown in the spectrum above, wikipedia again says  


  metal-halide lamps have high luminous efficacy of around 75 - 100 lumens per watt...


Putting it all together: say you have a 2000 lumen metal halide light with the spectrum above. The number of "green" photons emitted per second can be found by google:
"2000 lumens / (82 lumens/watt) * 800/6000 * 630 nm /planck's constant /speed of light"
which says 1019 "green" photons/second.