There are two possible solutions for your case:
1. Use the RadScheduler AppointmentDataBound event handler and add a custom class to the appointment object.
sender, SchedulerEventArgs e)
The attached screenshot shows where this custom class is applied.
2. To use color translator in order to convert the hex color value in System.Drawing.Color
Color _color = System.Drawing.ColorTranslator.FromHtml(hex);
and use the suggested article.
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