Get number of system colors

Use GetDisplayColors function for this problem. This function returns number of bits on point of screen. For example, 16-65535, 8-256, 4-16.

function GetDisplayColors: integer;
var 
  tHDC: hdc;
begin
  tHDC:=GetDC(0);
  Result:=GetDeviceCaps(tHDC, 12)*GetDeviceCaps(tHDC, 14);
  ReleaseDC(0, tHDC);
end;