Here are some things to consider:
* Visibility: Bright colors like red, orange, or yellow are easier to see, especially in low light conditions.
* Your dog's coat: A contrasting color to your dog's fur will make it easier to spot them.
* Your dog's personality: Some dogs might prefer a specific color.
* Practicality: A collar should be safe and comfortable for your dog.
Ultimately, the best color for your border collie's collar is the one that you and your dog like best!