So the rule that you posted only works for the color property because the selector for the rule doesn’t have a high enough specificity. That is, there is this rule in the theme’s style.css file:
#logo h1, #logo-sticky h1 {
font-size: 22px;
line-height: 1.4;
font-weight: bold;
margin-top: 10px;
}
So the use of the ID logo gives the rule a higher specificity than the use of the class name site-title, which is why the font size remains 22px. What you should do is use the same selector as the theme, and as long as your rule comes after the theme’s rule, then your rule will override the theme’s rule:
#logo h1 {
font-size: 50px;
color: #0000ff
}
If you are using the theme’s Custom CSS option (Appearance → Customize → Theme Options → General Settings → Custom CSS), then your rules will come after the theme’s style.css file, so it’s just a matter of using the same selector as the rule you are trying to override.
Do not edit the theme’s style.css file (or any other theme file). If you do, then your changes will be lost the next time you update the theme.
If you are trying to figure out what existing rules are in place for a particular element, then learn to use a web debugging tool like Firebug (a Firefox extension) or Chrome Developer Tools (which is already a part of Chrome). I personally like Chrome DevTools, it’s as easy as right-clicking on an element and doing a inspect element.