In Colorado, you can be held liable if your dog bites someone, even if your dog has never bitten before. Colorado has a strict liability law for dog bites, which means the dog owner is responsible, regardless of the dog's previous behavior.

However, there are some exceptions. For example, you may not be liable if the bitten person was trespassing or provoked the dog.

If your dog has bitten someone, it's important to consult with an attorney as soon as possible. They can help you understand your legal rights and options, and represent you in court if necessary.

Your dog could be considered dangerous if it bites, acts aggressively, attacks other animals, chases people or animals, jumps on people, or barks excessively. If your dog exhibits any of these behaviors, it's important to seek professional help from a dog trainer or behaviorist. 

In Colorado, even if your dog was provoked, you can still be liable for a dog bite that causes serious bodily injury. This is because Colorado has a strict liability law for dog bites, which means the dog owner is responsible, regardless of the circumstances.

However, there are some exceptions to this rule. You may have a defense if you can prove that the injured person was trespassing, ignored posted warning signs, knowingly provoked your dog, or was a professional working with dogs (like a veterinarian or dog trainer).

An attorney can help you understand your rights and options if your dog has bitten someone.