Azalea Bud Freeze Damage
Notice the brown bud in the center of the photo. This is a flower bud that has been killed by a late April freeze. The yellow leaves though are normal on a white of pink azalea coming out of the winter.
If you have noticed a lighter than normal bloom on some of your azaleas, this is probably due to late freezes in March or April killing the flower buds before they bloomed. You may notice many cases in which there are flowers on the bottom of the plant where the buds were more protected. The amount and pattern of damage will be variable due to the variety of the plant and the location and microenvironment of where the plant was growing.
The species most likely to show this type of damage are Indica azaleas, but there are other sensitive varieties such as Glacier and some pink varieties. The good news is that the plants will recover without any intervention other than normal pruning but remember this can happen every year depending upon the spring weather.