The Tour Detail page “BOOK NOW” button color comes from Customizer >> Layout and Design >> Main Colors >> Primary Color (this is the default button background color) and Secondary color (this is the hover color).
If you want to override this, please go to Appearance >> Customize >> Custom CSS and JS >> Custom CSS bo and add the CSS below inside the CSS box (replace #c02127 with your desired color hex value):
.btn.btn-info {
background: #c02127;
border-color: #c02127;
}