Red, White, Purple, and Green). However, they're not always as easy to see when you've got a lot of people running around in a boss fight. They're great for explanations, don't get me wrong, but they rarely last for the duration of the fight (unless your guild is just that good, at which point I doubt you'd need the visual aids in the first place).
So what do you use when you need a big, visible focal point for your raiders?
Archaeology! Specifically, you use the Druid and Priest Statue Set, a rare project from the Night Elf school of archaeology. While it doesn't last nearly as long as the flares do, it does provide a huge beam of light over the user. Now, I know Blizzard didn't intend for archaeology (or any secondary profession) to be required for endgame, but it seems to me that this, while not explicitly required, is definitely a huge boon to raiders everywhere.
During any boss fight where a mechanic requires you to stack up, this can be activated to give people a clear signal of where to go. For any healer about to use an AoE heal in an emergency situation, this can be included in a macro. Raiders can see the beam of green and white light and know to run in for healing.
The only major drawback is that it's on a 15 minute cooldown, so it would realistically only be usable once per person per boss fight. But even so, it's not a bad idea to have it available for a quick visual aid.
I'm sure most raiders could be easily taught that whenever they see the light, run into it. Every time. Stacking together to avoid a mechanic or recieve an AoE heal can save you a lot of headaches over time.
To get this, you'll have to collect 100 Night Elf Fragments, which may include up to 3 keystones. Since you can pretty much start archaeology with Night Elf projects, you've got a decent chance of getting this together. And you can always farm it to death, if necessary.