A photo pigment is a molecule in a photoreceptor cell (a rod or cone) that can trap the energy in visible light and convert it into a signal that can trigger a nerve cell. When the nerve cell is triggered, your nervous system knows that light is hitting that particular photoreceptor.
The way pigments usually work is by having a 3-dimensional structure with a special arrangement of chemical bonds. When light of the correct wavelength hits the pigment, the electrons in the bonds are moved around, and this changes the 3-D structure of the pigment. When the pigment changes shape, it causes the photoreceptor cell to send a nerve impulse to the brain. The pigment molecule then
has to return to its original 3-D shape to be able
to be stimulated again.
Different pigments absorb light of different wavelengths. The cone photoreceptors in our eyes have 3 different pigments that absorb 3 different wavelengths, so the range of wavelengths that our cones can 'see'
is quite broad -- ranging from blue to red with a peak at green (remember -- the spectrum goes Red-Orange-Yellow-Green-Blue-Indigo-Violet -- ROY G BIV). This range of the visible spectrum that the cones can absorb is called the photopic response. The range of the spectrum that the rods can absorb is called the scotopic response, and it is much narrower -- really just in the blue-violet range. It is much narrower because rods only have
one pigment, and it absorbs blue-violet light.
Rods are much more sensitive in low-light
conditions than cones are.
So at nighttime when there is very little light, we mostly see with our rods. But because the scotopic response is very narrow, we can only see blues, not the entire spectrum of visible light. This is why everything
has a blueish tinge when you are trying to see in
very dark conditions.
I hope this answered your question!