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Most people don't know what ores actually look like IRL so videogames just go with old conventions (iron is gray = iron ore is gray). Very few people know what uranium ore looks like (thankfully), but most people associate radioactivity with neon green plutonium rods so that's what most radioactive stuff is now. These conventions might seem a bit misleading but they're not bad per se. Neon green stuff is more awesome than boring yellow lumps of what might as well be sulfur. And having brown ore turn into gray iron plates while grayish ore turns into brown plates is just counter-intuitive.