Hi VT,
Thank you for using Syncfusion products.
Please find the sample to assist your requirement of changing the brush color of highlighted day cell from the following location,
http://www.syncfusion.com/downloads/Support/DirectTrac/107735/CalendarSample-1037356624.zip
In the above sample, we have customized the CalendarDayButton style and defined a key 'AccentBrush', changing the value of the key 'AccentBrush' will change the brush color of highlighted day cell.
Please let us know whether the sample assists your requirement.
Please let us know if you have any queries.
Regards,
Mageshyadav.M