Metallic colors can be selected in the color picker by choosing Color libraries and choosing the Pantone metallic color library. To reproduce those colors the work must use those inks on press. Or if you simply want the color but not the metallic effect, then choose spot to process when exporting a PDF from InDesign.
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Sure, those exist only in printed form, using special inks. You can't make them with CMYK or RGB. If you want to have metallic colours on professionally printed work, you need to collaborate with your printer.