You have an old-style switch loop configuration, with no neutral in the box, just hot, switched, and ground. This means that you'll have to take your "smart" light switch back and find one that does not need a neutral wire.
First off -- the only guarantees found in North American electrical code are that neutrals are white or grey (but not all whites are neutral) and grounds are green, green/yellow striped, or bare. Things that are neither ground nor neutral can be any other color -- the reason black, red, and to a lesser extent blue are common is because those colors are what you get with NM cable.
HOWEVER: in a large building like your condo, the wiring will likely be done in conduit instead, which means that you won't find a ground wire (as the conduit subs in for that) and the other wires can be any color whatsoever other than green, white, or grey.
I suspect the yellow wires are hot, by the way, as there are two of them to the same screw on the switch -- that implies that the orange is switched, going off to whatever that switch controls. It's a lot harder to tell on the other switch, but perhaps the red is hot and the brown is switched? Note that a standard single pole dimmer or switch doesn't care which way it is hooked up -- the two brass terminals or black pigtail wires on the switch or dimmer are interchangeable and equivalent.
By the way, when you do put a dimmer in -- it'd be a travesty if you threw a nice, spec-grade switch with wire-clamp plates out in the garbage and put the cheapest builder-grade trash dimmer in in its place. Get a decent spec-grade dimmer from the likes of Lutron, Leviton, or Cooper; it'll stand a good chance of being move-in-ready for whoever you sell your condo to if/when you move out. (The switch is good as a spare, too, if say the next person who moves in hates dimmers, or you need it for something else.)
Best Answer
Odds are excellent that you have an additional bare wire and cable sheath out of sight up the ceiling. And that your "yellow" wire is actually "White, but old and yellowed."
This is a crappy install, in that the cable sheaths should have been coming all the way into the "old fixture as junction box" and if they don't make it to the hole, that obviously wasn't happening, and the next thing in line wasn't grounded, (and I doubt the first thing was given how short that ground wire is cut,) unless that cable has just retreated into the hole after you removed the fixture. Bare copper or green are grounds.
It's marginally possible but highly unlikely that the ballast in one fixture was running a tube in a separate fixture (where yellow would be a reasonable likelihood as a real color.) If so, you're going to need to replace that wire anyway.
For cables used in North America, the conductors are black, white, bare, then black, white, red, bare, etc - i.e. you won't find a yellow wire in a normal household cable. You might in a conduit. But the yellowed/aged white wire is a much more likely case.