A:  This is a custom component that overrides the DrawCell.  It changes the brush and pen colors, then calls the inherited DrawCell method, then resets the colors.

unit New_cal;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  Forms, Dialogs, Grids, Calendar;

type
  TEMS = class(TCalendar)
  private
    { Private declarations }
    FAllowHighlighting: boolean;
  protected
    { Protected declarations }
  public
    { Public declarations }
    procedure DrawCell(ACol, ARow: Longint; ARect: TRect; AState: TGridDrawState); override;
    constructor Create(AOwner: TComponent); override;
  published
    { Published declarations }
    property AllowHighlighting: boolean read FAllowHighlighting write FAllowHighlighting;
  end;

procedure Register;
 

implementation

procedure Register;
begin
  RegisterComponents('Samples', [TEMS]);
end;

constructor TEMS.Create(AOwner: TComponent);
begin
  AllowHighlighting := true;
  DefaultDrawing := false;
  inherited create(AOwner);
end;

procedure TEMS.DrawCell(ACol, ARow: Longint; ARect: TRect; AState: TGridDrawState);
var
  OldCanvas: TCanvas;
begin
  inherited DrawCell(ACol, ARow, ARect, AState);
  if AllowHighlighting and (gdSelected in AState) then begin
    OldCanvas := TCanvas.create;
    try
      OldCanvas.brush.color := canvas.brush.color;
      OldCanvas.font.color := canvas.font.color;
      canvas.brush.color := clWhite;
      canvas.font.color := clBlack;

      inherited DrawCell(ACol, ARow, ARect, AState);

      canvas.font.color := OldCanvas.font.color;
      canvas.brush.color := canvas.brush.color;
    finally
      OldCanvas.free;
    end;
  end;
end;

end.