Thanks for writing that up, Womble, and for showing the tags.
Personally, I use the Ritchey sprays only for
very temporary - marking the ones I've treated in the pen, so no one gets two doses, that temporary. It's not rain proof, so a few days is all you can rely on.
We use the Carrs Billington ones, which are the best I've tried, they will reliably show for two months or more on white wool. (So can be used for scanning marks, and will still be visible at lambing.). Sounds like the Nettex are similar. We usually use orange, red, green, blue or purple for lasting marks. Yellow for more transient, as it's less visible to start with. You do need to shake the can vigorously before spraying, or you can get clods of powder, which do rub off on other sheep.
Marks on heads never seem to last long at all.