アポイントメント型セル(AppointmentCell)のRenderer.FillColorプロパティでバーの色を設定できますが、テンプレートに配置したアポイントメント型セルのFillColorプロパティを設定すると同一行のアポイントメント型セルのバーは同じ色になります。
同一行のアポイントメント型セルに異なるバーの色を設定するには、実行時に複数のアポイントメント型セルを生成し、それぞれをセルに設定します。

[Visual Basic]
Imports GrapeCity.Win.CalendarGrid

Dim today As DateTime = DateTime.Today

' Appointment型セルの設定
Dim appointmentCellType1 As New CalendarAppointmentCellType()
appointmentCellType1.Renderer.FillColor = Color.Yellow

Dim appointmentCellType2 As New CalendarAppointmentCellType()
appointmentCellType2.Renderer.FillColor = Color.LightGreen

' CalendarGridにAppointment型セルを配置
GcCalendarGrid1.Content(today).Rows(1).Cells(0).CellType = appointmentCellType1.Clone()
GcCalendarGrid1.Content(today).Rows(1).Cells(0).Value = "項目1"
GcCalendarGrid1.Content(today).Rows(1).Cells(0).ColumnSpan = 3

GcCalendarGrid1.Content(today.AddDays(4)).Rows(1).Cells(0).CellType = appointmentCellType2.Clone()
GcCalendarGrid1.Content(today.AddDays(4)).Rows(1).Cells(0).Value = "項目2"
GcCalendarGrid1.Content(today.AddDays(4)).Rows(1).Cells(0).ColumnSpan = 3

[C#]
using GrapeCity.Win.CalendarGrid;

var today = DateTime.Today;

// Appointment型セルの設定
CalendarAppointmentCellType appointmentCellType1 = new CalendarAppointmentCellType();
appointmentCellType1.Renderer.FillColor = Color.Yellow;

CalendarAppointmentCellType appointmentCellType2 = new CalendarAppointmentCellType();
appointmentCellType2.Renderer.FillColor = Color.LightGreen;

// CalendarGridにAppointment型セルを配置
gcCalendarGrid1.Content[today].Rows[1].Cells[0].CellType = appointmentCellType1.Clone();
gcCalendarGrid1.Content[today].Rows[1].Cells[0].Value = "項目1";
gcCalendarGrid1.Content[today].Rows[1].Cells[0].ColumnSpan = 3;

gcCalendarGrid1.Content[today.AddDays(4)].Rows[1].Cells[0].CellType = appointmentCellType2.Clone();
gcCalendarGrid1.Content[today.AddDays(4)].Rows[1].Cells[0].Value = "項目2";
gcCalendarGrid1.Content[today.AddDays(4)].Rows[1].Cells[0].ColumnSpan = 3;