This would just be confusing for players. We already rely on item color for quick ballpark identification - this would make it harder.

items already change color in some special cases, most notably when in water.

If anything, rusty items should be light red (or orange?) - the same color as a rusty potion (which is the same a copper-hewn girdle). As there are no weapons of this color, this won't hurt visual identification.

Provided you don't use brown (which is used pretty much across the board for wood, on weapons especially, or leather) I don't think there is much potential for confusion here - as soon as you step on the item, it's described as "rusty", so it's clear where the coloring comes from. Like Ln I would favor the game's "official" color for rust; that of the rust monster.

Edited in consideration of feedback provided. Thanks all for the input. :)

Yep, as long as it isn't a commonly used color already, it should be fine - I quite like it. Precedent also exists in certain prefixed weapons, so it makes perfect sense.

Hmm, I think that if this is ever implemented the color of rust could be configurated On/Off with a the ":=" option command with something like Rust_Color = True/False, for those who want to avoid confusion.