You don't say how long these have been in the ground - planting too deeply, planting in soil with too high of a PH, planting in poorly draining soils can cause the light color you describe, or chorosis. It would help to know your approximate soil PH too.

Rhododendrons and azaleas do not tolerate as much fertilizer as many other plants ( here where our soils are acidic I never fertilize). If planting soil is very poor and sandy or if fresh sawdust has been added to the soil, it is advisable to fertilize. If the leaves are light green or yellow, but not yellow with green veins, the plant should respond to a light application of fertilizer. About one-fourth to one-half the rate recommended for other kinds of plants should be adequate. (A split application, one-half in March or early April and one-half in mid-May). Scatter the fertilizer lightly around outer edge of the root ball. Do not apply fertilizer in late summer or fall as the plant may be stimulated into growth and be killed during the winter. If in doubt, do not fertilize. (more rhododendrons and azaleas die from too much fertilizer than from none at all)

Rhododendrons may have yellow leaves with green veins - This is chlorosis and is usually caused by lack of available iron, possibly because the soil is not acid enough. Chelated iron sprays on the foliage will help, but if alkaline soil is the problem, it should be corrected. A soil test is helpful in diagnosing. Lime from a building foundation, building debris, excessive fertilization, excessive dryness or excessive wetness are some possible causes of chlorosis - (Anything that might damage roots can cause chlorosis, the roots that remain are insufficient to provide the plants nutrients)

Yellowing of leaves falls into the following categories.

Yellowing of a leaf between darl green veins is called chlorosis. Many conditions can be responsible. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency or magnesium deficiency. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer.

Yellowing of leaves sufaces, often with brownish burned areas, occuring on leave that are more exposed to sun, is caused by more sun exposure than the plant is able to tolerate.

Yellowing and dropping of leaves is normal toward the end of the second summer on the small-leaved lepidote rhododendrons. These should have dense enough habit that this doesn't matter.

Uniformly yellowish-green leaves is often just the need for more nitrogen. This will be more noticeable in the full sun. Some less sun tolerant varieties will always be light green in full sun.

Yellowing of leaf edges has been noted in gardens where sandy soil conditions or root competition with other plants caused insufficient soil moisture and nutrients.

See the link below for more information.

Here is a link that might be useful: Yellowing of rhododendron leaves

BTW, some cultivars have naturally light(er) green leaves and some, Scintillation in particular, always look like it's stragling from chlorosis while it's its natural appearance.

My Scintillation plants never have light green leaves. They are always dark and shinny. If Scintillation gets too much sun it gets sun scald which causes brown areas on the leaves. It does best in some shade.