Meyer lemon trees (Citrus x Meyeri) typically have waxy, dark-green leaves all year round. While yellowing of the leaves is not always a problem, investigate the source of the yellowing if you suspect the leaves should not be yellowing at this time.
Meyer lemon trees are evergreen, but just before the spring flush of new leaves, Meyer lemons will drop old leaves. Since they're about to be discarded, the leaves have often lost their color and gone partially or completely yellow. Meyer lemons may also flush in fall, and you may notice yellow leaves then, too.
Zinc deficiency will cause yellow stripes to appear near the leaf veins, but will also be accompanied by stunted twigs and fruit drop. Lack of manganese in the soil will cause the area between the veins to turn yellow, and leaves will start to die and shed. Incorporating plenty of compost at planting and mulching with compost helps prevent these deficiencies.
When citrus mites suck the juices from Meyer lemon leaves, the top of the leaves becomes white or yellow-speckled. Check the undersides for light webbing and pinprick-sized creatures moving about. Treat with insecticidal soap, especially if the Meyer is grown indoors. Mite populations can get out of control in sheltered indoor locations with no natural predators.