By default visual editor formatting buttons works only for post content. Whereas post title uses the same default font type, style, color etc pre-defined in your theme’s CSS file.
Custom Title is an awesome WordPress plugin that lets you customize post title. This way admin can style each blog post title heading using native WordPress functionality.
Start by installing and activating Custom Title plugin in WordPress
Upon activation the plugin adds a visual editor on post title box. You can customize the title while creating new posts or editing the old ones.
How It Works? Currently the plugin works by adding markup on post title box which helps a non-programmer person to style post title using TinyMCE formatting buttons.