Grid-lines in "print" to png appear black always, not grey

[Tim Pierce]

Octave 4.0.0

code:

    figure

    plot( 1:100 )

    grid on

    print( 'test.png', '-dpng ','-S500,500', '-color' )

In https://engineeringliberty.wordpress.com/2011/10/07/making-great-plots-in-octave-and-matlab-too/ 

I can see that the second graph down has an example of nice grey (as per the window) 

The figure looks like this in Octave

It prints to test.png like this

I have been through all the gca settings I can see, relevant. There's a LOT.
The printed plot respects none of them

If anyone could help diagnose this it would be appreciated

Tim