For leaves that have no obstructing hairs on the surface, one can assume that a lighter green surface is reflecting more light, hence is absorbing less or emitting more light, than a darker one.
Not all leaves are the same shade of green. Variations in shade of green within a plant community, even on a single plant, are quite common, and are influenced by many factors (Examples of green shades in communities: conifer community, light gap of a tropical rain forest, and altiplano of Chile).
A plant that is grown in full sun (sun leaves) often produces leaves having a lighter shade of green than the same plant grown under very low light intensity (shade leaves). In part this is attributable to different proportions of the two chlorophyll pigments, because chlorophyll b is very abundant in shade leaves, to improve light-capturing capability of the chloroplast. Thus, shade leaves, having adaptations for capturing the low intensities of sunlight, are not designed for optimal photosynthesis when given full exposure to direct sunlight. A shade leaf is, in effect, a cheap unit of photosynthetic area, costing less to make and maintain than a sun leaf.
Thick, nonsucculent leaves of dicotyledons tend to be dark green (Example: Pouteria splendens). The dark color is attributed to the high absorptance of sunlight by the dense chloroplasts and low reflectance from the cell walls, and sunlight not used by the upper leaf are channeled between the cells to the underlying leaf cells, where the remainder of the light is captured and utilized. Thick succulent leaves, if not masked by surface wax, tend to be light green, probably because the watery cells are large, hence chlorophyll concentration per surface area of the leaf is relatively low (Example: jade plant).
Upper (adaxial) and lower (abaxial) surfaces of a leaf often are different shades of green. If different, the upper surface is always darker (Examples: Ilex cornuta and star jasmine). This is expected because, for many leaves, the concentration of chloroplasts (chlorophyll and carotenoids) is much higher in the upper portion of the leaf, hence is lower in the lower portion. A vertically oriented leaf, as well as the typical monocotyledonous leaf, has no difference in shade for the two surfaces, because tissues on both sides of the leaf are rich in chloroplasts (Examples: phyllode of an acacia and Persoonia).
For individual plants, young leaves often are a lighter shade of green than fully mature and aging leaves on the same plant (Examples: leaves of ivy, Podocarpus, and coffee). The older leaf has its full complement of photosynthetic pigments and cell wall maturation is complete, both which contribute to the darker tone.
When a leaf ages, it may "turn" yellow, orange, or red during the final stage of senescence (Examples: Tabebuia, Asian cherry, and sweetgum). This results when the chlorophyll molecules are dismantled and nitrogen (N) is translocated out of the leaf, revealing underlying pigments in that leaf that were masked by the green chlorophyll. Many leaves appear yellow before death sets in. The remarkable fall coloration occurs for some deciduous trees and shrubs of temperate regions, especially common forest trees of eastern North America. Translocation of nitrogen from senescing leaves is an important strategy to reuse this important nutrient in other parts of the plant.
Chlorosis is a classic symptom of nutrient deficiency in which the leaf is yellow or mottled yellow-green (chlorotic leaf of citrus). Most common deficiencies for this pale coloration are due to low nitrogen, low magnesium, or unavailable iron in the soil. A single atom of magnesium (Mg) occurs in the middle of each chlorophyll molecule. Iron (Fe) is required by the cell to make chlorophyll.
An etiolated plant cannot complete synthesis of chlorophyll when grown in darkness, so that the leaf is yellowish or whitish, due only to the presence of yellow carotenoids. In etiolated plants, there are no normal chloroplasts, because in the constant darkness the typical membrane and protein portions of the organelle cannot be made.