Thanks for reaching out to us. I can help you out here. Looks like you are close.
I play around a bit with the css and came up with these two selectors:
/*Change Font Color of Days with Events */
.tribe-mini-calendar-grid-wrapper .tribe-mini-calendar div[id*="daynum-"] a {
color: red;
}
/*Change Font Color of Days without Events */
.tribe-mini-calendar-grid-wrapper .tribe-mini-calendar div[id*="daynum-"] .tribe-mini-calendar-no-event {
color: blue;
}
Drop that into the stylesheet or through a plugin such as Simple Custom CSS.
I could not find this as a selector at least in my mini calender’s source code in my browser:
tribe-mini-calendar-day-link
Let me know if that works for you. Might have to tweak the css a bit to get everything how you would like.