Every time I change the footer widget title color in the customizer, a new post is generated in the database with the following code. The post status = trash. Is it safe to delete these from the database? When are these posts removed automatically from the database?
I believe these are changesets set by WordPress. They work as drafts for your Customizer options, so you can save the options without publishing them.
I believe they should delete themselves automatically over time, but you should be safe to delete them manually. Just be sure to backup your database first, just in case.