They commonly turn yellow if they have too much water. Is the soil wet? If it is, give it some time to absorb the water you've given it. Feel the soil with your finger to tell. In general, unless otherwise marked, water them only once a week.
There are a range of reasons:
-not enough light
-too much fertilizer
-nitrogen or iron deficiency
-some types of pests such as aphids or mites
-too much water; stopping the roots from receiving enough oxygen
Tad bit of extra help: the soil should be springy to the touch, not crumbly and dry, and you finger shouldn't come away more than a tiny bit wet.
The death of the cloraphyll. Then the true color of the leave structure shows.
Chlorophyll is a pigment. It's not alive.
its either too much water or to little.