This would match arrows and other shapes, which are by default red. PS: sorry for using an old version. This is due to bug 468229.
This is a bug in the KImageAnnotator library that Spectacle was using for annotations; you can report the issue at https://github.com/ksnip/kImageAnnotator/issues/ However in the upcoming 23.04 release, we ported away from that library and wrote our own, and in that release the issue is fixed and we default to red text.