How change the alpha value of a specific color in a 32 bit TBitmap?

I need to change the value of the alpha component when a pixel contains a specific color for a TBitmap of 32 bits, I know about the ScanLine property to access the bitmap data, but i can't figure out how change the alpha component of each pixel.

This is a basic implementation

First you need define a record to hold the ARGB structure

  TRGB32 = record
    B, G, R, A: byte;
  end;

Then you must define a array of TRGB32 to cast the Scanline and get and set the values.

Check this sample method

procedure SetAlphaBitmap(const Dest: TBitmap;Color : TColor;Alpha:Byte);
type
  TRGB32 = record
    B, G, R, A: byte;
  end;
  PRGBArray32 = ^TRGBArray32;
  TRGBArray32 = array[0..0] of TRGB32;
var
  x, y:    integer;
  Line, Delta: integer;
  ColorRGB : TColor;
begin
  if Dest.PixelFormat<>pf32bit then  exit;

  ColorRGB:=ColorToRGB(Color);
  Line  := integer(Dest.ScanLine[0]);
  Delta := integer(Dest.ScanLine[1]) - Line;
  for y := 0 to Dest.Height - 1 do
  begin
    for x := 0 to Dest.Width - 1 do
      if TColor(RGB(PRGBArray32(Line)[x].R, PRGBArray32(Line)[x].G, PRGBArray32(Line)[x].B))=ColorRGB then
        PRGBArray32(Line)[x].A := Alpha;
    Inc(Line, Delta);
  end;
end;

Also you can take a look to this unit that i wrote to manipulate 32 bit bitmaps