Hello,

You should choose page title https://prnt.sc/p4a8m2, in this case, all the page titles would be changed. If you need to change the color of the title only for the blog page add this custom CSS to the Theme Settings > Custom CSS > Global:

body.blog .title-size-small .entry-title, .title-size-small h1 {
    color:green;
}

Add any styles you need.

Best Regards