The only reason plating will have different colors depends on purity. Then whatever other metals make up the mix. Example being Krugerrands are alloyed with copper giving them a pinkish hue. American eagles are alloyed with silver giving them a brighter whiter finish. Canadian Maples are pure giving them the appearance of natural yellow gold.
If an experienced person could tell an item was plated just by looking at it then gold jewelry buyers wouldn't be spending $30k on x-ray spectrometers.
By the way that Sigma you posted a pic of doesn't perform a density test for verification. You need to upgrade to the Sigma Pro because there's tons of fakes out there that can show good on the original Sigma.....RIP