Best Method of marking Whins/Briars while spot spraying them?

Hi all,

Have young whins and briars about 3 ft high which I intend treating with Grazon Pro in the next week or so....(waiting for a few dry evenings).They are on hilly ground so I am getting the lend of a Quad sprayer tank with handheld lance with around 20 foot of a pipe run on it.Will put it on my quad.

There were strong whins and briars in isolated bunches on this ground but I got them dug out by a track machine a few years ago which was expensive but a great job.As tends to be the case with 'weeds' .....they are starting to come back better than ever now so I want to stop them in their tracks....no pun intended!

Is over 10 acres of dryish hilly ground where they are but if you can imagine there are only new isolated regrowth bunches of Whins or Briars coming every 30 or 40 foot or so. In an area of a 100 foot radius from sitting on the bike you might only have 7 or 8 different bunches to lance Spray.

I'm trying to think of an effective cheap way for myself to put some simple marker on the sprayed stuff as they are done so I'm not covering some bunches twice.

I was thinking would a bright colour like yellow of the Aerosol Can Sheep sprays be as good as any for it?They are about a fiver a can. I wonder would that stay on them if it rained.

Or is there a cheaper, easier way of marking the sprayed bunches?

Thanks for anyone's thoughts.