Can protoss counter mass marines in the open?

So I didn't bother to try more than just this, but this will work for you:

• 30 Marines vs 3 Colossus

With relatively little micro (I only moved the first Colossus back at 50% health) I was able to keep all three Colossus alive against Marines with both Stim (which you have to use manually) and Combat Shield.

Edit: I also found

• 4 Sentries and 2 Templar are enough, but that requires more micro
• 1 Mothership and 3 Templar (Vortex into Storm)

Marines do little damage (each). Get a mix of sentries, high templars and colossi. Shoot force fields ahead of the marine army to block them off (always works best at choke points), move sentries back and have them cast Guardian Shield to reduce the incoming damage to HT and Colossi. Get the range upgrade for the Colossi and put them in the back. Psi Storm with HT. If you got the Hallucination upgrade for the sentries, cast it and let them take some damage. You will have far less units, so the microing should be far easier for you. The other player, in turn, will have to rely a lot more on CPU targeting. Unless you are playing TLO, of course.

ALTERNATIVELY, and this might be risky, get a bunch of Dark Templars. IF (and only if) your adversary doesn't bother bringing a raven along, they will slice and dice them marines up before they knew what hit them. Considering that he will be spending lots of minerals on units, he will probably spend his first gas in core reactors instead of starports. A DT "rush" is slow and risky, but if you scout him and see marines racking up, you could go for it. -> Pylon -> GW -> CC -> Pylon -> TA -> DS with probes and a couple zealots to hold a rush in between. Ain't no expert in BOs though.

Cheers.