The links on the booking calendar initially inherit the color of the links under Setup > General Layout.
If the system detects that there’s not enough contrast between the link color and the background color of the service it will automatically update the link color for improved contrast and readability.
You may still want the booking calendar links to be a different color.

To do this, you can add some CSS to the global CSS file.
.CourseBooking a,
.ClosedDay a,
.HiddenBooking a,
.AdminSlotBooked a,
.AdminSlotNew a,
.AdminSlotNote a,
.AdminSlotCheck a,
.AdminSlotNoteCheck a,
.AdminSlotPaid a,
.AdminSlotPaidNote a,
.AdminSlotPaidCheck a,
.AdminSlotPaidNoteCheck a,
.AdminSlotPaidConfNoteCheck a,
.AdminSlotPaidConfCheck a,
.AdminSlotPaidConfNote a,
.AdminSlotPaidConf a,
.AdminSlotPaidBellNoteCheck a,
.AdminSlotPaidBellCheck a,
.AdminSlotPaidBellNote a,
.AdminSlotPaidBell a,
.AdminSlotPaidBellNote a,
.AdminSlotPaidBellCheck a,
.AdminSlotPaidBellNoteCheck a,
.AdminSlotBell a,
.AdminSlotBellNote a,
.AdminSlotBellCheck a,
.AdminSlotBellNoteCheck a,
.AdminSlotConf a,
.AdminSlotConfNote a,
.AdminSlotConfCheck a,
.AdminSlotConfNoteCheck a,
.AdminSlotConfBell a,
.AdminSlotConfBellNote a,
.AdminSlotConfBellCheck a,
.AdminSlotConfBellNoteCheck a,
.AdminSlotNew a,
.AdminSlotNewNote a,
.AdminSlotNewCheck a,
.AdminSlotNewNoteCheck a,
.AdminSlotNewPaid a,
.AdminSlotNewPaidNote a,
.AdminSlotNewPaidCheck a,
.AdminSlotNewPaidNoteCheck a,
.AdminSlotNewPaidConf a,
.AdminSlotNewPaidConfNote a,
.AdminSlotNewPaidConfCheck a,
.AdminSlotNewPaidConfNoteCheck a,
.AdminSlotNewPaidBell a,
.AdminSlotNewPaidBellNote a,
.AdminSlotNewPaidBellCheck a,
.AdminSlotNewPaidBellNoteCheck a,
.AdminSlotNewBell a,
.AdminSlotNewBellNote a,
.AdminSlotNewBellCheck a,
.AdminSlotNewBellNoteCheck a,
.AdminSlotNewConf a,
.AdminSlotNewConfNote a,
.AdminSlotNewConfCheck a,
.AdminSlotNewConfNoteCheck a,
.AdminSlotNewConfBell a,
.AdminSlotNewConfBellNote a,
.AdminSlotNewConfBellCheck a,
.AdminSlotNewConfBellNoteCheck a,
.tblRoster a.contrastTextLight:link,
.tblRoster a.contrastTextLight:visited {
color: #000!important;
}