How do I interpret the heart rate monitor?

There are three statuses that the heart monitor can display, as mentioned in the question: normal, quick and orange, and slow and blue. Normal is self-explanatory, it means the monster is not in any special state.

The orange, quickened heartbeat signifies that the monster is in rage mode. This is triggered by receiving damage and, while rage mode is more frequent when the monster's health drops, it doesn't have a 100% uptime. While in rage mode, the monster becomes more aggressive, faster, and deals more damage.

The blue, slower heartbeat signifies that the monster is exhausted. This is triggered by the monster tiring itself out from constantly attacking, and can be quickened by using impact weapons, which apply exhaust damage. Once a certain amount of exhaust damage has been applied, the monster's stamina will drop by a chunk and, if low enough, will leave the monster exhausted. An exhausted monster is slower than normal, may stand still while doing nothing, and will often try to hunt prey or eat carcasses to recover stamina.