What do the colours of the scatterplot3d actually show?

Hello,

Please could anyone tell me the significance of the graded red-black color
when using scatterplot3d?

At first I thought it was just going from the very 'near' to the back of the
chart area, but as you can see from the pictures some of the points in the
back are bright red. The description from ?scatterplot mentions that
hightlight3d "points will be drawn in different colors related to y
coordinates"... but as you can tell from the plot is isn't true either; with
come short points being black as well as red...

<http://r.789695.n4.nabble.com/file/n4689148/Rplot.png>

any help would be fantastic, I'm very very new to R

Cheers,

Jo