They're "Marley" tiles (vinyl composition tiles to give them their correct name). As said above, they normally respond best by far to a floor stripper, but mixed at about half the strength (or less) than you'd use for actually stripping sealer off if it was there.
But that brings another question, is the floor finished in any way? Could throw the whole conversation in a different direction!
I'd be very wary about using stuff like citrus concentrate on these, because solvents can damage them. You're not using orange juice, you're using some unpredictable solvents. If you applied a d-limonene based product to them I'm sorry but you've probably screwed them. I learned this the hard way, luckily on my mum's old kitchen floor shortly before it was ripped up and replaced!
Be very wary about over-scrubbing as you'll start to remove the dense, compacted surface layer of the tiles which is smoother and shinier than the "biscuit" inside it. On a good conditioned surface, a black pad is way too much, blue should be the heaviest you'd need and I'd start of with red.
If they're already worn then it's too late to worry about that, so in which case I wouldn't even do the job unless they wanted the floor finished with a good quality sealer afterwards. If you leave these tiles in a very worn state with no sealer on them they will just suck up soiling and look pants in no time at all. An already trashed and worn floor will not come off too badly from a black pad, it will lose some more material but at least you'll get it clean! You'll see the colour of the tile material washing up in your slurry whilst scrubbing.
Have fun whatever you do
p.s. you shouldn't be paying any more than about £15 or so for 5L of a decent floor stripper. Don't listen to BS marketing