[R] rgl package don't draw points with a color over points with another color

[rrichte2 at fh-lausitz.de]

  I want to plot points with different colors to show different 
selections of points in a 3d plot.

My problem is that if I plot a red point at a location where already a 
blue point was plotted, than the point is still blue.
Is there a parameter or so which can be used to draw over a existing point?

Example:
 > require(rgl)
 > open3d()
 > plot3d(a<-c(1,2,3,4,5),a,a,type="l", col="blue")    # draw line
 > plot3d(a<-c(1,2,3,4,5),a,a,type="p",col="blue",add=TRUE)    # mark 
points at the line
 > plot3d(a<-c(3),a,a,col="red",type="p",add=TRUE)    # mark a point 
with another color

thanks
roland