Is there a correlation between wavelength and voltage or frequency output
Not enough to be useful, even if light of only one wavelength is involved.
If you have many wavelengths simultaneously impinging on the sensor, it is utterly hopeless to sort them out after they interact with the photodiode. The filter MUST go between the sensor and the light source.
Narrow filter bandwidth = better discrimination = more expensive. Go with what you can afford and see if it works. Perhaps green cellophane will work well enough.