There are different places where the calendar colors should occur:

• day
• week
• month
• agenda

The example sets them like this:

		.violet {
			--dhx-scheduler-event-background: linear-gradient(180deg, #D071EF 0%, #EE71D5 100%);
		}

		.green {
			--dhx-scheduler-event-background: linear-gradient(180deg, #12D979 0%, #1ECDEB 100%);
		}

		.yellow {
			--dhx-scheduler-event-background: linear-gradient(180deg, #FFB725 0%, #FFBB25 31.25%, #FAEA27 100%);
		}
	</style>

Version 7 of the scheduler makes color customization so much easier:
https://docs.dhtmlx.com/scheduler/custom_skins.html

Thus, we should update the way we handle colors to use CSS variables.