procedure TForm1.GradientRect (FromRGB, ToRGB: TColor; Canvas: TCanvas);
var
  RGBFrom : array[0..2] of Byte; { from RGB values }
  RGBDiff : array[0..2] of integer; { difference of from/to RGB values }
  ColorBand : TRect; { color band rectangular coordinates }
  I : Integer; { color band index }
  R : Byte; { a color band's R value }
  G : Byte; { a color band's G value }
  B : Byte; { a color band's B value }
begin
  { extract from RGB values}
  RGBFrom[0] := GetRValue (ColorToRGB (FromRGB));
  RGBFrom[1] := GetGValue (ColorToRGB (FromRGB));
  RGBFrom[2] := GetBValue (ColorToRGB (FromRGB));
  { calculate difference of from and to RGB values}
  RGBDiff[0] := GetRValue (ColorToRGB (ToRGB)) - RGBFrom[0];
  RGBDiff[1] := GetGValue (ColorToRGB (ToRGB)) - RGBFrom[1];
  RGBDiff[2] := GetBValue (ColorToRGB (ToRGB)) - RGBFrom[2];

  { set pen sytle and mode}
  Canvas.Pen.Style := psSolid;
  Canvas.Pen.Mode := pmCopy;

  { set color band's left and right coordinates}
  ColorBand.Left := 0;
  ColorBand.Right:= canvas.ClipRect.Right-Canvas.ClipRect.Left;

  for I := 0 to $ff do
  begin
  { calculate color band's top and bottom coordinates}
  ColorBand.Top := MulDiv (I , canvas.ClipRect.Bottom-Canvas.ClipRect.Top, $100);
  ColorBand.Bottom := MulDiv (I + 1,canvas.ClipRect.Bottom-Canvas.ClipRect.Top, $100);
  { calculate color band color}
  R := RGBFrom[0] + MulDiv (I, RGBDiff[0], $ff);
  G := RGBFrom[1] + MulDiv (I, RGBDiff[1], $ff);
  B := RGBFrom[2] + MulDiv (I, RGBDiff[2], $ff);
  { select brush and paint color band}
  Canvas.Brush.Color := RGB (R, G, B);
  Canvas.FillRect (ColorBand);
  end;
end;

public
  { Public declarations }
  procedure GradientRect(FromRGB, ToRGB: TColor; Canvas: TCanvas);

GradientRect (clBlue, clBlack, Canvas);

Paint;