Electrical – Brown, yellow, and orange wires in a wall switch

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.)