You can do this by setting a style in your paragraph tag. For example if you wanted to change the font size to 28px.
<p style="font-size: 28px;"> Hello, World! </p>
You can also set the color by setting:
<p style="color: blue;"> Hello, World! </p>
However, if you want to preview font sizes and colors (which I recommend doing) before you add them to your website and use them. I recommend testing them out beforehand so you pick a good font size and color that contrasts well with the background. I recommend using this site if you wish to do so, couldn't find anything else: http://fontpreview.herokuapp.com/