The theory:

A computer image is made with pixels. All pixels are coded in RGB, e.b. R for red, G for green and B for blue. With a mix of all those 3 colors, you can produce any colors. By example, 100% Red + 100% Green = Pure Yellow . In windows, all RGB colors are coded in octals [0-255], where 255 is 100%. With this information, a programmer can make a software to change anything in a picture.

This is an example:

The color of the eyes:

Before you can color the eyes, you just have to define the work zone. For this picture of Avril Lavigne, I defined this zone:

X > 145 < 291 , Y > 250 < 270 ,  X  < 168 >212

Step 2, Detect the color to change:

Avril have blue-green eyes. This color have exactly the same level of blue and green.  The formula to detect this color is:

Red < 82, Green > 16, Blue > 16

Step 3, Change the color

I tried different kinds of colors and levels but the best, the most natural was 13.7 % more of blue. This is the technique:

If Blue < 255-35 Then Blue = Blue + 35 ( 255-35 is the maximum of blue level )

To keep the darkness of the original eyes, add:

If Red > 35 Then Red = Red - 35
If Green > 35 Then Red = Red - 35

You can also change any colors of your choice, by example you can boost only the green ( to make green eyes.)

This is an example of programmation for Delphi:

( first define 2 Images Box (Image1 & Image2) )

Procedure TForm1.Button1Click(Sender: TObject);
var
  c:LongInt;
  color:longint;
  red,green,blue:Byte;
  x,y:integer;
  bobo:Boolean;

begin
  for y:=0 to my do // my is the height of Image1
  begin
    for x:=0 to mx do  // mx is the width of Image1
      begin
      c:=Image1.Canvas.Pixels[x, y];  // C = The current pixel
      blue := GetBValue(c);
      green := GetGValue(c);
      red  := GetRValue(c);
      bobo:=False;
      if x in [168..212] then bobo:=True; // exclusion zone
      if  (x>145) and (x<291) and (y>250) and (y<270) and (bobo=False) then // in the eyes ?
      if  (blue>16) and (green>16) and (red<82) then  // blue-green color ?
      begin
        cc:=35; // 13.7 %
        if blue<=255-cc then blue:=blue+cc;  // the blue color is boosted
        //if green>=cc then green:=green-cc; // add if you want to keep the darkness of the original
        //if red>=cc then red:=red-cc; //  ''  ''  ''
      end;
      color:=RGB(red,green,blue); // Color = the new color
      Image2.Canvas.Pixels[x,y]:=color; // we put the pixel
    end;
  end;
end;

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