Yes but it's the same pool of metal used for all 3 rings. They didn't split the mithril into multiple parts to use for 3 different rings, they literally tossed what they had into the same pool of metal, and there's no reason why a 'copper mixed with gold' alloy would yield three different colored results. If it's pinkish, then it's pinkish when split three ways, not one being pink another being copper colored and another being gold. What you're trying to explain doesn't make sense in how one pool of molten metal yields three different results, because we literally see them creating a single alloy out of the mithril, not three separate alloys.
Well it's not a mistake that they showed one pool of metal with the mithril dropped in right? Not a mistake that they melted the dagger as a whole as one piece instead of separating the gold and silver parts right? If this whole sequence is a mistake then sure, I can agree with you, but it's clear that they don't care enough about the entire sequence to really show how the ingots were differentiated. It's as magical as Arondir's buzzcut.The show doesn't bank on people not paying attention. Mistakes happen hence why Game of Thrones had a starbucks cup and House of the Dragons forgot to digitally remove fingers and left the green tape around the fingers.
Like, people can call this shit out as nitpicks, and sure, that's what these are. But it's details that have been clear they haven't been putting effort in making sense in their own world, and it's a criticism that still holds true to the final episode for the most important part of the show - the creation of the rings. Even if these are nitpicks, even if it's movie magic, it's not excusable by merely saying 'it was a mistake like the Starbucks cup in GoT'. No, it's not comparable to the Starbucks cup at all here. These are deliberate sequences presenting something that simply isn't realistic or sensible. These are scenes that were watched over many times by many people who work on the show, going through multiple iterations of approval. And this final result is what was approved. It's not a random exposition scene we're talking about here, it's quite a pivotal moment.
As someone who works in film and vfx, it really boggles the mind how this got passed into the final cut. It comes down to two explanations - either hundreds of people fell asleep at the job and missed it, or the more sensible reason that who ever approved the sequencing ultimately didn't care about the realism to have it fixed, be it pushing mistakes through because they had no time for reshoots or meeting a deadline or whatever it may be.