Most archers select their string colors based on the color of their bow. And there's nothing wrong with that. But there are a few things to consider when choosing your next string color:
Tuning: Different colors will result in a different weight string. This is because of how the string material is manufactured. Some colors (such as white) have very little wax while others (like fluorescent colors) have a much higher wax content. The more wax, the heavier the resulting string. And strings of different weight will tune differently. To learn more check out Archery Path.
String blur: String blur should also be considered. Some people see some string blur colors better than others. And this should be tested in varying lighting conditions as well. However you don't need to buy a bunch of strings to test this. Strips of construction paper of different colors can be used to simulate what you will see with your string blur and what colors work best for you.
Twist direction: Some archers have trouble remembering which direction to twist the string to adjust brace height. Choosing a multi-color string is an easy way to more easily see which way the string is twisted. That can be used as a reference to figure out which way to twist to raise or lower the brace height.
Now you have more information to make a more educated choice for your next string!