These trees have good color and a full canopy of leaves. This is not to say that some leaves will not be damaged in some way. This is natural and nothing to be concerned about. Also as trees grow some branches will become increasingly shaded and the leaves on those branches may turn yellow. This is a natural process and is different from leaves that are chlorotic from insect, disease, nutrient deficiency or some other problem.
Plants vary tremendously in what constitutes a healthy leaf color.
Some are quite pale and others a very deep green. You need to know what is normal for the species you are observing. Yellow or spotted leaves or those with noticeable damage are cause for suspicion. Yellow leaves are usually referred to as chlorotic and may be caused by many different factors such as nutrient deficiency, insects, disease etc. You must be aware of cultivars that have leaves that are not green.