Wow, 200 broilers is a daunting thought.
With last year's batch from the hatchery, when I ordered CX and Rocks, they came mixed together in the box and they had marked the CX with a blue marker. It worked pretty well, and it was enough for me to get them separated out of the box, but it only lasted about a week or so and faded rather fast. If you want to go that route I would use a good dark, bright color, and be sure to reapply regularly. You can also get tiny little rubber bands, like the kind they use on kid's braces. I use those a lot for new chicks, then upgrade to colored zipties later. Just don't forget to upgrade the sizes as they get bigger.
Another thought is Blu-Kote. It's an antiseptic spray (any farm store should have it). It's a great antiseptic, but it leaves a permanent dark purple stain. That might work well too. If you get it on a feather it will NEVER come off. I have a hen that still has a dark purple butt from a pecking incident months ago. Just spray a little on a Q-Tip and wipe it on the wing or tail feathers, not the fluff. It should stay until the baby feathers molt out.
Jen