function DitherColor(AColor: TColor; AAlpha: Integer): TColor;
{ Dither a 32-bit color(with alpha value) to TColor.}
var
  iRGB: TRGB;
begin
  iRGB := TColor2TRGB(AColor);

  if (iRGB.r = 255) and (iRGB.g = 255) and (iRGB.b = 255) then begin
    iRGB.r := AAlpha;
    iRGB.g := AAlpha;
    iRGB.b := AAlpha;
  end else begin
    iRGB.r := Trunc(AAlpha / 255 * iRGB.r) + (255 - AAlpha);
    iRGB.g := Trunc(AAlpha / 255 * iRGB.g) + (255 - AAlpha);
    iRGB.b := Trunc(AAlpha / 255 * iRGB.b) + (255 - AAlpha);
  end;

  Result := TRGB2TColor(iRGB);
end;

function DitherColor(AColor: TColor; AAlpha: Integer): TColor;
{ Dither a 32-bit color(with alpha value) to TColor.  This functions
  correctly for all colors except for clwhite. }
var
  iRGB: TRGB;
begin
  iRGB := TColor2TRGB(AColor);
  iRGB.r := Trunc(AAlpha / 255 * iRGB.r) + (255 - AAlpha);
  iRGB.g := Trunc(AAlpha / 255 * iRGB.g) + (255 - AAlpha);
  iRGB.b := Trunc(AAlpha / 255 * iRGB.b) + (255 - AAlpha);
  Result := TRGB2TColor(iRGB);
end;