An off-topic question regarding basic Java Programming

[tr4nquility]

Code:

gd.setColor(new Color( red[counter],green[counter],blue[counter]));
          gd.drawLine(start.x, start.y, end.x, end.y);

Have you tried to also set the alpha value of the line's color?

Yes, I have...

Code:

gd.setColor(new Color( red[counter],green[counter],blue[counter], 255 ));

Using maximum alpha value 255 still no good. The color itself indeed will adjust to its own alpha value, but it is contained within the main panel / frame which has its own opacity. It the end, the opacity of line will be adjusted with the opacity of its container...
If we set the container's opacity to 0f, then no line will be showed...

P.S : my final objective is to get this line drawn in fully transparent background ^_^. The cursor image will act like a "laser pointer" used in presentations... so when the pointer is moved, we will have a line drawn to imitate a highlighter.