На форму поставьте таймер, и в нем напишите

 

procedure TForm1.Timer1Timer(Sender: TObject);

const

SySColorArray: array [0..13] of  Integer = (COLOR_ACTIVEBORDER,

COLOR_ACTIVECAPTION, COLOR_APPWORKSPACE, COLOR_BACKGROUND,

COLOR_BTNFACE, COLOR_BTNTEXT, COLOR_CAPTIONTEXT,

COLOR_INACTIVEBORDER, COLOR_INFOTEXT, COLOR_MENU,

COLOR_MENUTEXT, COLOR_WINDOW, COLOR_WINDOWFRAME,

COLOR_WINDOWTEXT);

 

ColorArray:array [0..10] of TColor = (clAqua, clBlack, clBlue, clYellow,clFuchsia, clGreen, clNavy, clRed, clSilver, clWhite, clSkyBlue);

Begin

SetSysColors(1, SySColorArray[random(13)], ColorArray[random(10)]);

end;