[R] filled.contour - color palette so z=0 ONLY is blue

[Laura Quinn]

I am trying to create a topographic map of an island - the filled.contour
function works fine except i am experiencing difficulty trying to
represent the sea properly. Basically I want the default colour blue for
any instance where z=0, if I simply use the default topo.color I get
shades of blue for z values up to 220 metres. Is there a way in which I
can specify terrain.color for all values of z>0 but blue for z=0 - or is
there another way of acheiving this simply?

Thanks,
Laura