You are correct that an adverbial precedes its verbal head, but here it is not a problem of word order, but one of sentence structure.
In a subject-predicate type Chinese sentence, there can be many predicates that come one by one, as long as they share the same subject.
Snowflakes, as big as cotton and as small as willow catbins, were falling down heavily, covering the world with a cloak of snow.
The subject here is 雪花(snowflakes) and it is followed by four phrases as predicates. In this kind of sentence, any (but not all) of the predicates can be absent without destroying the sentence structure, but once the subject is absent, the structure is destroyed (or the sentence sounds like a riddle).
The order of these predicates: we cannot tell which predicate must come first. This depends on the content of each of them (they may follow some spatial, logical, temporal or other orders). Length may also be a consideration: a longer, more structurally complicated phrase is usually put at the end.
In your question, the structure is:
S:它的眼睛 + P:(P1:红红的 + P2:像一对红宝石)
Predicate P2 is both longer and more complicated, and is less logically related to the subject itself: being red is an inherent attribute of the eyes themselves, but being like rubies is not the eyes' attribute but a description we made about them.
Finally, to help you identify structures (between a predicate or adverbial-head structure), remember that in the latter case the head must be present, while any of the predicate can be absent. Example:
It's eyes were red. 它的眼睛红红的。
It's eyes were like a pair of rubies. 它的眼睛像一对红宝石。
but in 他快樂地笑 (He laughed happily) :