Numbers 21:9

A copper snake. He was not told to make it of copper, but Moshe said: “The Holy One, Blessed is He, called it “ נָחָשׁ “ (“snake,”) so I shall make it of “ נְחֹשֶׁת “ (“copper”). The terminology is well matched.

And Mosheh made a serpent of brass, and set it upon a place aloft; and it was, when a serpent had bitten a man, and the serpent of brass was gazed at, and his heart was intent upon the Name of the Word of the Lord, he lived.

AND MOSES MADE ‘NECHASH NECHOSHETH’ (A SERPENT OF BRASS). “He was not told to make it of brass. But Moses said: ‘The Holy One, blessed be He, called it a nachash (serpent); therefore I will make it of nechosheth (brass)’ — since the one word resembles the other [in sound].” This is Rashi’s language, based upon the words of our Rabbis. But I do not understand this, for the Holy One, blessed be He, did not mention to Moses [the word] nachash, but said to him, Make thee ‘saraf’ (a fiery serpent)! But the intention of the Sages is that Moses was guided by the substantive name thereof [which is nachash; and saraf is merely a particular description of a certain kind of serpent — a “fiery” serpent — hence he made the saraf which G-d mentioned to him from nechosheth (brass), corresponding with its substantive name].
It appears to me that the secret of this matter is that this is one of the ways of the Torah, every deed of which is a miracle within a miracle. Thus [the Torah] removes injury by means of the cause of the injury, and heals illness by means of the cause of the sickness, as the Rabbis have mentioned in connection with [the verse], and the Eternal showed him a tree, and as occurred with the salt that Elisha [cast] into the water. Now it is a well-known medical principle that all people bitten by poisonous creatures become dangerously ill when they see them, or [even] when they [only] see their likeness, so that if people who have been bitten by a mad dog or other mad animals look into water, they see there the image of the dog or the attacker, and [this] can [cause them to] die, as is written in medical books and mentioned in the Gemara of Tractate Yoma. Similarly doctors protect them from [people] mentioning in their presence the name of the animal that bit them, [and they forbid people] to mention it at all, because their minds cling to this thought and do not turn away from it altogether until it causes their death. [The doctors] have already mentioned that it is an empirical fact, amongst the wonders of reproduction, that if the urine of a person bitten by a mad dog is put in a glass receptacle after he has become rabidly sick, there will appear in that urine the likeness of the young of small dogs. And if you pass the liquid through a cloth and filter it, you will not find any trace of them at all; but if you return the liquid to the glass bottle, and it remains there …

נחש נחושת, after Moses had understood what G’d had intended with His command, he agreed to make this replica of a snake out of copper and not out of gold. Copper would be more likely to induce thoughts of remorse as they would associate its colour with the sensation of burning.