Fans of Alternity

Yes, since Sally Anne has used blue to respond to the code, it makes sense that maybe red and black have distinct meanings and blue is the response either way. Could red and black be different priorities of message? Like, red is very important or an emergency, and black is less important? Maybe they were using black before to practice the code, but it was only to convey that they had minor messages (like, here is a chocolate frog I got you). That's maybe why, in the first instance of code use, Pansy says that it's definitely not red.

Now would be the first time that red is actually invoked.