MDS just had some red ones in grass. That is genius!! It shouts out to us....but just looks like long grass to dogs (red/green "color blindness").
I guess it depends on the "red" of the red dirt....but I would think red or green would be good.
While I've not seen a single dog as much as look at the markers, I think a contrasting color - to dogs' vision - could be more of an attraction.
JMHO,
diane