The numbers next to the topics?

I am curious. As a new forum member I notice there are numbers next to the topics. Some are red, some are black and some are shaded grey. What is the significance of these color differences? I assume the numbers correspond to the number of new posts for the topic. Am I correct?

The software is tracking what you’ve read, so the numbers indicate the number of posts in a topic, or the number of posts in a topic since you’ve read it, or the number of posts since you replied to a topic etc. and all displayed differently.