There is no way to chnage a hover color on posts titles free of charge. A few themes do provide for changing font color in the Blog Title and/or optional Tagline and/or links but not many. In the majority of themes font changes to font families, colors and size in the blog title, tagline, menu, post titles, page titles, categories and tags, widget headings, comments and links cannot be done without purchasing an annually renewable custom design upgrade and doing the required CSS editing. http://wpbtips.wordpress.com/2009/03/24/color-customisable-themes/
There are three ways to change fonts on WordPress.com free hosted blogs.
(1) Changing font families, colors and size in text in posts or pages either post by post or page by page by coding into the HTML editor. http://en.support.wordpress.com/editors/font-size-color/
(2) Changing font colors in text in posts or pages either post by post or page by page using the #4 icon (Select text color – change the text color) in Row 2 the Visual editor. http://en.support.wordpress.com/visual-editor/#row-2
(3) Changing all font families, colors and sizes throughout the whole blog by purchasing an annually renewable Custom Design upgrade http://en.support.wordpress.com/custom-design/