A photo would be really helpful, but... Sometimes lace bug damage can cause this appearance. The leaves look sickly yellow-white but the veins remain green. Check the leaf undersides. A speckled appearance from lace bug excrement is a sure sign that this is the problem.

More likely is chlorosis. This is, despite what many people think, not a diagnosis but a general term for the failure of leaves to produce sufficient chlorophyll. It can have a number of causes, alone or in combination: soil pH too high, too wet, too dry, too compacted, insufficient nitrogen and or iron, etc. The only sure way to tell is a soil test. Feeding without knowing the cause can do more harm than good.

I agree 100% with aka.

There are a number of things you can do:

Make sure the plant has excellent drainage. You may have to ditch around outside the drip line and run it to a good drainage area.

Get a soil test. Some things to look for are:
pH, Ca, Fe, K, Mg, P

Applying powdered sulfur is fairly benign, unless you live in an area that has very acidic soil. That is very unusual. Sulfur is very slow acting, but very long lasting.

Do not fertilize unless you know you need it. Now is a bad time to fertilize azaleas. If you decide to use a general fertilizer (one containing nitrogen) wait until next spring around bloom time and then use a fertilizer with organic nitrogen like HollyTone and never use it more than once a year and then at half the rate on the package.

Here is a link that might be useful: Chlorosis

I have yellow leaves on a couple of my rhododendrons. I have noticed some very tiny specks of white "insects" flying around the bushes and have now sprayed with a general plant spray against insects. Did this yesterday, so hope that it works.

The yellow leaves with green veins are definitely not caused by insects. They are strictly a nutritional problem.

By spraying with insecticides indiscriminately you may give yourself a bad case of Lace Bugs. One of the main ways to control Lace Bug is with natural predators, i.e. spiders. That is the reason Lace Bug is a problem in sunny locations is because spiders don't like sunny locations. Unfortunately some of the most severe problems in the yard start by using a chemical that causes more harm than good. There is a very long list of such chemicals. The chemical companies love people who do this. Every time they create a new problem, they buy more chemicals.

The best advice is to identify the cause of any problem. Then determine if it is worth doing anything about. If so, then find a solution that doesn't cause more problems. Definitely don't go by the information from the company or store selling the products. Common sources of good information are your county agent and local horticultural clubs.