Hi Kristin-
The color of the site title is automatically generated based on your color palette in the Customize > Colors menu. However, if you’d like to pick one manually, you can do that with a little bit of custom CSS.
Navigate to Customize > Advanced > Custom JS & CSS and paste the following line into the Custom Theme CSS field:
h1.site-title a { color: purple !important; }
You can replace “purple” with the color of your choice, or use a hex value like #000000 or rgb value like rgb(0,0,0).