I marked timber for years and years. It's the job nobody wants to do, unless they are fallers and get paid well for carrying cans of paint instead of a heavy saw and accessories.
Make sure you get that butt mark (the one at stump level) as close to the ground as possible and get the paint into crevices or scars where it can't be easily rubbed off by a skidder tire. Bunchers/processors cut very low stumps so that requires marking low. You've also got to have a bit of knowledge to identify how a tree is going to fall or if it will be possible to get the tree on the ground without falling additional trees, that you might want to leave. It's the job that can't be done perfectly. Additional trees will most likely have to come down during operations because of hung up trees and road, skid road, and landing construction.
Loggers like a lot of paint on the tree. Markers like less because it means less paint to carry.
Use a specific color and maybe ask around to see what the common paint scheme is. Around here on federal lands, orange is for leave trees, blue is for cut trees. On state lands, it looks like blue is for leave trees and sometimes fallers get confused. I've had to deal with having a couple of acres of leave trees cut by confused fallers. It isn't fun.
Use tree marking paint. It's made to last a few years and doesn't get sucked up by the bark. Logging supply stores probably carry it and then online forestry supply outfits do carry it.
And then, during logging, check up on the operations yourself. Walk through the area frequently after work has stopped for the day. Ask questions.
We call that portion of the butt log that is cut off because it has excessive sweep, a long butt. Keep an eye on that. Sometimes the cutter goes overboard and longbutts portions that are actually merch. And, this means you might want to peruse some forestry books on tree defects. I am "out west" so have an old copy of the Idaho log scaling book. There are probably some local books in your area showing various rots and defects and how to ID those.
It takes a while to learn and that's why there are foresters out there.