Why does "is" take an adjective and not an adverb?
Look at these as verbs (sound, smell etc) as describing a state (as "to be" does). In this case, we have an observed state, rather than a definite state.
Compare it is green with it looks green.
In the first case, we are certain that "it" is green.
In the second case, my observation leads me to believe that it is green (but I am not certain -- it may be difficult to tell due to light).
The meaning is very close.
Now compare "he plays well" with "he is well". The meaning is totally different. This is because the verb "play" does not describe an observed state, but an action, so you cannot substitute "be"/"is".