color1:=clRed;
color2:=clYellow;
Panel1.Color:=color1+color2; ????

function ContrastColor(AColor: TColor): TColor;
const
TolerSq = 16 * 16;
begin
if Sqr(GetRValue(AColor) - $80) + Sqr(GetGValue(AColor) - $80)
  + Sqr(GetBValue(AColor) - $80) < TolerSq then
  Result := (AColor + $7F7F7F) and $FFFFFF
else
  Result := AColor xor $FFFFFF;
end;

function MixerColors(FromColor, ToColor: TColor; Rate: Single): TColor;
var
 ra, rb: byte;
begin
 RA := round(Rate * 255);
 RB := 255 - RA;
 result := (((Lo(FromColor) * RA + Lo(ToColor) * RB) shr 8) or
           (((Lo(FromColor shr 8) * RA + Lo(ToColor shr 8) * RB) shr 8) shl 8) or
           (((Lo(FromColor shr 16) * RA + Lo(ToColor shr 16) * RB) shr 8) shl 16));
end;

uses
 Math;

function MixColors(Color1, Color2: cardinal): cardinal; overload;
var
 R, G, B : byte;
begin
 R := Min(GetRValue(Color1) + GetRValue(Color2), 255);
 G := Min(GetGValue(Color1) + GetGValue(Color2), 255);
 B := Min(GetBValue(Color1) + GetBValue(Color2), 255);
 Result := RGB(R, G, B)
end;

function MixColors(Color1, Color2, Color3: cardinal): cardinal; overload;
var
 C: cardinal;
begin
 C := MixColors(Color1, Color2);
 Result := MixColors(C, Color3)
end;

// OnShow, ComboBox1.OnChange, ComboBox2.OnChange, ComboBox3.OnChange
procedure TForm1.ChangeColor(Sender: TObject);
var
 Color1, Color2, Color3: cardinal;
begin
 Color1 := $FF shl (ComboBox1.ItemIndex * 8);
 Color2 := $FF shl (ComboBox2.ItemIndex * 8);
 Color3 := $FF shl (ComboBox3.ItemIndex * 8);
 Color := MixColors(Color1, Color2, Color3)
end;

function GetMixColor (c1, c2: TColor): TColor;
begin
Result := RGB (
                (GetRValue (c1) + GetRValue (c2)) div 2,
                (GetGValue (c1) + GetGValue (c2)) div 2,
                (GetBValue (c1) + GetBValue (c2)) div 2
              );
end;

// OnShow, ComboBoxN.OnChange, TrackBar.OnChange
procedure TForm1.ChangeColor(Sender: TObject);
var
 Y, C1, C2, C3: cardinal;
begin
 Y := MulDiv($FF, TrackBar1.Position, TrackBar1.Max);
 C1 := (Y shl (ComboBox1.ItemIndex * 8));
 C2 := (Y shl (ComboBox2.ItemIndex * 8));
 C3 := (Y shl (ComboBox3.ItemIndex * 8));
 Color := MixColors(C1, C2, C3);
 Caption := IntToHex(Color, 8)
end;

unit ColorsMixing2;

interface

uses
 Windows, Graphics, Controls, Forms, ComCtrls, ExtCtrls;

type
 TForm1 = class(TForm)
   procedure FormShow(Sender: TObject);
   procedure FormCreate(Sender: TObject);
 private
   tbR, tbG, tbB: TTrackBar;
   Panel: TPanel;
 end;

var
 Form1: TForm1;

implementation

{$R *.dfm}

uses
 Math;

function MixColors(C1, C2: cardinal): cardinal; overload;
var
 R, G, B : byte;
begin
 R := Min(GetRValue(C1) + GetRValue(C2), 255);
 G := Min(GetGValue(C1) + GetGValue(C2), 255);
 B := Min(GetBValue(C1) + GetBValue(C2), 255);
 Result := RGB(R, G, B)
end;

function MixColors(C1, C2, C3: cardinal): cardinal; overload;
var
 C: cardinal;
begin
 C := MixColors(C1, C2);
 Result := MixColors(C, C3)
end;

procedure TForm1.FormShow(Sender: TObject);
var
 CR, CG, CB: cardinal;
begin
 CR := tbR.Max - tbR.Position;
 CG := (tbG.Max - tbG.Position) shl 8;
 CB := (tbB.Max - tbB.Position) shl 16;
 Panel.Color := MixColors(CR, CG, CB);
 Caption := ColorToString(Panel.Color)
end;

procedure TForm1.FormCreate(Sender: TObject);

 function CreateTrackBar(AHint: string; ATabOrder: integer): TTrackBar;
 begin
   Result := TTrackBar.Create(Self);
   with Result  do
   begin
     Parent := Self;
     Orientation := trVertical;
     Align := alLeft;
     TickMarks := tmBoth;
     Width := 40;
     Max := 255;
     Position := 255;
     Frequency := 5;
     PageSize := 51;
     ShowHint := True;
     Hint := AHint;
     TabOrder := ATabOrder;
     OnChange := OnShow
   end
 end;

begin // FormShow
 tbB := CreateTrackBar('Blue', 2);
 tbG := CreateTrackBar('Green', 1);
 tbR := CreateTrackBar('Red', 0);
 Panel := TPanel.Create(Self);
 Panel.Parent := Self;
 Panel.Align := alClient;
 Panel.Caption := ''
end;

end.