I doubt that the player in question not being able to differentiate between colors was the issue here, it was more likely simply not paying attention, but that is beside the point.
The object can be identified by a 'look' command, and the player would do well to routinely check anything altar-looking that way anyway before stepping on them, because that's the way they get to know its alignment. The situation is the same: hey look, this here has the icon '_', it might be a chaotic altar, let's check. Whether the list of "might be chaotic altars" include only lawful altars, neutral altars and chaotic altars or additionally some other somewhat similar special objects (the elemental anomalies) does not matter much.