It can be a subtle distinction, but the tooltips do explain it.
It’s the same new/unread distinction we make between these two kinds of topics:
- this is a new topic with unread posts (New tab)
- this is an old topic with unread posts (Unread tab)
… except expressed at the post level:
- these are new unread posts (Blue number)
- these are old unread posts (Grey number)
You might want to know that there are:
- 50 new unread posts in this topic (blue 50)
- 100 old unread posts in this topic (grey 100)
This gives you an idea of how much real “newness” or “freshness” you can expect to get if you enter the topic at your current read position.
If a topic is not getting any new posts, the distinction is definitely irrelevant. In other words, there are no more episodes of the Sopranos being recorded, so you have a constant single number of episodes left to watch at any given time. But if it’s say, Adventure Time, then you have two numbers: number of episodes left in this season, and number of episodes left all time. Maybe the Sopranos case (you are entering long-dead topics) is true enough of the time that grey/blue does not matter.
But if it is an active topic, knowing:
- Here’s how many “old episodes” I need to watch just to finish the season.
- Here’s how many “new episodes” have been recorded after the current season.
The intent is to give you a sense of how far behind you are falling, and how much the topic is growing:
- If the grey number is large and the blue number is small, the topic is barely growing.
- If the grey number is large and the blue number is large, the topic is growing faster than you can read it.
- If the grey number is the only number: the topic is dead, there is no growth.
- If the blue number is the only number: the topic is all growth.
It kind of bothers me with only the blue number that you have no idea if these are really “new” replies you’re being informed about. They could be unread replies from 5 years ago…
But then again to be honest I rarely run into the grey numbers in practice. Either I read to the end, or I just don’t care about the topic and tap out before the 4 minute default auto-track interval. You’d see very different results if you changed the default tracking settings to, say, auto-track every topic you enter.