The rationale for this is that if you need to post a long text post, it should be in the form of a blog post instead. I agree with you that it's not really adapted for a meta post.

Maybe post color is based on some get_text_post_color method that applies to self posts and comments, where the color depends on comment karma. Given that self posts like the OP are votable as if they were a normal link post, their comment karma value is probably 0.

I don’t know pg’s reasons for making self-posts light gray, but you can fix problems like that with the bookmarklet Zap Colors: https://www.squarefree.com/bookmarklets/zap.html

I'm using "Hacker News Enhancement Suite" Chrome extension - it fixes multiple problems, including this one.