Hmmm..., looks strange. In FormCreate you add a point at x=2, y=11 in normal color. In Button1Click you add another point at the same location in red color, and in Button2Click you add another one, now in normal color again. In the end you'll have multiple points at the same location. I guess this is not what you want.
In order to colorize an already existing point you should use the method SetColor which is available for most series; it gets the point index and the new color as parameters. If the new color is clDefault (or clTAColor) then the color is restored to the series color.
See attached demo.