Mix two TColors

// This function mixes two bytes According to value of TRANS
// The value of TRANS is between 0 (result then will be equal to FG)
// and 255 (result then will be equal to BG)
function MixBytes(FG, BG, TRANS: byte): byte;
asm
  push bx  // push some regs
  push cx
  push dx
  mov DH,TRANS // remembering Transparency value (or Opacity - as you like)
  mov BL,FG    // filling registers with our values
  mov AL,DH    // BL = ForeGround (FG)
  mov CL,BG    // CL = BackGround (BG)
  xor AH,AH    // Clear High-order parts of regs
  xor BH,BH
  xor CH,CH
  mul BL       // AL=AL*BL
  mov BX,AX    // BX=AX
  xor AH,AH
  mov AL,DH
  xor AL,$FF   // AX=(255-TRANS)
  mul CL       // AL=AL*CL
  add AX,BX    // AX=AX+BX
  shr AX,8     // Fine! Here we have mixed value in AL
  pop dx       // Hm... No rubbish after us, ok?
  pop cx
  pop bx       // Bye, dear Assembler - we go home to Delphi!
end;

// Here we mix R,G and B channels of our colors separately.
// The value of T is between 0 and 255 as described above.

// As you know, TColor value is 4 bytes length integer value where
// low byte is red channel, 2nd byte is green and 3rd byte is blue

function MixColors(FG, BG: TColor; T: byte): TColor;
var r,g,b:byte;
begin
  R := MixBytes(FG and 255,BG and 255,T); // extracting and mixing Red
  G := MixBytes((FG shr 8) and 255,(BG shr 8) and 255,T); // the same with green
  B := MixBytes((FG shr 16) and 255,(BG shr 16) and 255,T); // and blue, of course
  Result := r+g*256+b*65536; // finishing with combining all channels together
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  b : byte;
begin
  b := 50; // Max 255
  Memo1.Brush.Color :=  MixColors(clLime, clNavy, b);
end;