Not a particularly nice way of doing it. But i use the bootstrap datepicker widget from the appstore and i noticed this widget has the ability to ‘disable’ certain days. These days get the following classes added to them “disabled” and “disabled-date”. You could style these with a different colour than the current gray colour if you’re not looking for gray. (i just left them on this by default)

But if you have knowledge about javascript (which i sadly don’t) you could probably easily add a similar function on top of disabled days if you want other specific days to receive a specific class (which you can then colour). Which would probably nicer than ‘abusing’ the disabled class for it.