Generally, no special effort is required to print in color assuming the printer is a color printer and is set to its native color mode. Windows will automatically convert color to black and white if the printer does not support color. If you need to change the color mode in code, you can access the DevMode structure of the printer driver. Example:

uses Printers;
procedure TForm .Button Click(Sender: TObject);
var
  Device : array[0..255] of char;
  Driver : array[0..255] of char;
  Port   : array[0..255] of char;
  hDMode : THandle;
  PDMode : PDEVMODE;
begin
  with Printer do begin
    PrinterIndex := PrinterIndex;
    GetPrinter(Device, Driver, Port, hDMode);
    if hDMode <> 0 then begin
      pDMode := GlobalLock(hDMode);
      if pDMode <> nil then begin
        pDMode.dmFields := pDMode.dmFields or dm_Color;
        pDMode.dmColor := DMCOLOR_COLOR;
        GlobalUnlock(hDMode);
      end;
    end;
    PrinterIndex := PrinterIndex;
    BeginDoc;
    Canvas.Font.Color := clRed;
    Canvas.TextOut( 00, 00, 'Red As A Rose!');
    EndDoc;
  end;
end;