It could be chlorosis, are they pale limey green with a darker vein color initially?

If your azaleas are chlorotic, you need to determine the cause in order to treat. The common cause is alkaline soil and/or water...do you know the approximate PH of your soil?

Azaleas planted too deeply will become chlorotic when surface roots are unable to function. Overwatering and overfertilization can kill enough roots that the remainder supply inadequate amounts of iron. Same with root weevils and nematodes.

What kind of mulch have you used? 6" may be deep - possily deep enough to smother roots. If it's a material containing something like sawdust that may pack down, 2" would be recommended; pine needles, shredded oak leaves, looser materials can be layered somewhat deeper.

Lacebug damage can also cause the leaves to take on this appearance. Check the leaf undersides for evidence of lacebug feeding - pitted/stippled areas, usually brown or tan in color. Neem extract in summer weight horticulture oil gives good control, but the already damaged leaves will not regain their deep green color.

One of the problems with a mulch layer that is too deep is that the roots, searching for oxygen and water, will begin to actually grow into the mulch. 3 to 4 inches of a coarse organic mulch is optimum. This is something for us to remember for all plants, including trees. Too much is not a good thing. Plant roots that are forced to grow up into mulch are not able to access soil minerals, and are highly susceptible to heat and cold stress.

Remember, too, that all of that mulch is retrieving rainfall and irrigation. Most of the benefit of a wonderful summer rain, for example, may be captured in all of that mulch, rather than where you really want it.