Discourse doesn’t like short posts because they seldom have contain any actual information. The recommended way to say “Thanks” is to click the heart button under a post.
Sorry that I’m only seeing this thread now. As Doug noted, the message background colors are explained here. This feature has been in place since 2004, and I don’t think there have been any recent changes to how it works. The last few versions of SpamSieve have instead tweaked the color of the message text.
Thanks. Had missed the fact and just happened to see the colors. I usually mainly look to see if a good message got tagged. Has never happened with Spam Sieve but has with Apple’s filter