Well, here's my first answer on Stack Exchange!
All light, not just visible light which we see, consists of many different electromagnetic waves. These are all part of the electromagnetic spectrum.
The "color" of the light we see is defined by the wave's wavelength.
Here is a continuous spectrum - it contains all the different wavelengths of the visible spectrum, producing a white light.
The light can be diffracted (split) into its individual wavelengths and this is exactly what happens when light is shined at a prism, crystal, water vapor (such as in a rainbow), diffraction gratings or, in your example, an almost closed eye.
When your eye is almost closed the eyelids form a small "aperture", or thin gap for the light to pass though (Bio people, please correct me if that was wrong). The eyeball causes the thin line of light to be split into its individual wavelengths, and the light is in essence split to different parts of your retina and not as a continuous mix and a single color as you'd normally see it. This lets you see it in a way that reveals its separate wavelengths.
Since you say the street lamp produces a white light, it is most likely a Mercury-Vapour lamp. This is a gas-discharge tube which produces light by exciting Mercury Vapor.
Here is the emission spectrum of Mercury:
Note that it does not produce all wavelengths in the visible spectrum, but your eyes won't see it in such detail as it'll be rather blurred - you'll need a spectrometer to view it in more detail.
I doubt the street lamp uses and Incandescent light bulb, but if it did you'd see a near continuous spectra like the top image.
By the way, a phosphor coating is sometimes put over Mercury-Vapor lamps as it gives a better color temperature which makes it look more natural and white, so the spectrum probably looks more complete with that on. If the street lamp you saw was Mercury-Vapor then it was probably coated.
I'm not sure if my explanation of light diffraction in the eye is correct. It could bd moisture, but I would appreciate the input if someone knows for sure. Also other corrections in general, too. Thanks!