how can i change the color of a pixel in my sprite

You'll need to convert the sprites texture into a color array, change the exact pixel in the array, and set the colors of the texture to the new array, then apply the changes.

Something like this... May not be a copy & paste solution, but I hope you get the idea.

 Colors[] pixels = mySprite.texture.GetPixels();
 for (int x = 0; x < mySprite.texture.width)
 {
     for (int y = 0; y < mySprite.textute.height(
     {
         if (x == expectedXCoord && y == expectedYCoord)
        {
             pixels[x * y] = Color.black;
        }
     }
 }
 mySprite.texture.SetPixels(pixels);
 mySprite.texture.Apply();

I'd recommend writing a custom shader that lets you control the color of certain pixels of a texture.