The rend special attack of (let's say) a cave troll deals additional damage against targets hit by both front claws. The description includes the following: "it latches onto the opponent’s body and tears the flesh".
Is it just color or does it mean I can't rend a skeleton?
Please provide official sources where possible.
Having flesh or not is not defined by the game as a term (no special quality, type, subtype, etc.). As a result, the rules cannot refer to it, unless they do so explicitly with something like "this feature does not affect incorporeal creatures, constructs, or undead described as being bone without flesh." The fact that none of these game terms is used means that it's just color.
Usually it's easy to tell proscription (text that tells you what you can and can't do) from description (text that gives an image for how an action could take place). In contrast with SevenSidedDie's comment, this seems very much intentional on the part of the authors.
In some places (e.g. spell descriptions) they're even marked by different formatting. But in other cases, words of permission (can, may, etc) typically denote rules, and they often (though not consistently, I'm afraid) use the second person. Frequently you'll also see meta game terms (types, classes, actions, features, etc.).
Flavor tends to simply be declarative, using action verbs unqualified by words of permission. Meta terms are avoided or not formatted as they usually would be (though sadly Wizards was less than consistent about that).
While this is not always as clear as one would like, for the most part there is rarely disagreement. No one thinks that it is a rule, for example, that Sorcerers must be jealous of Duskblades' martial prowess, even though Player's Handbook II states it. Being able to tell the difference is an important part of understanding the rules.
The primary idea behind a rend is that the critter in question gets a hold in two spots and pulls them apart as violently as possible. So if you're fighting a skeleton, that has the damage reduction to reflect being scant, you grab two bones and wrench them apart like a perforated loose leaf sheet. So since the cave troll's natural enemies and prey are almost exclusively living things, flesh would be the most operative word.
