Leaves on plants can turn from green to light green or yellow for a number of reasons, many of which are related to the care of the plant. But certain diseases or insects can also cause plants leaves to change color.
All leaves or needles on a plant, evergreen or not, will eventually grow old and fall off. If you notice a few leaves here and there turning light green or yellow, then that's usually the case.
Overwatering a plant can cause leaves to turn pale green or yellow and then drop.
Sometimes simply moving a plant can cause it stress. As the plant adjusts to its new place, it will make adjustments related to light and temperature. During this period the plant is likely to lose some of its color and leaves.
Shifts in temperature where the plant is can affect its color. If a plant is too hot or cold where it's located, its leaves may become lighter in color.
Too much or too little fertilizer can drastically affect a plant. A nitrogen deficiency can also cause leaves to turn light green.
Insects and Disease
Insects feeding on a plant, either on its leaves or its roots, can cause leaves to turn light green, as can diseases common to the plant.