Seems this is based on a Russian novel. Why else would someone choose to put emphasis on two colors that can not be distinguished from each other in monochrome drawings.
Translator's discretion. Also, Merla is characterized related to Gold so might as well use Silver for him. I don't know how to word it but it feels much better to use Silver as it can be related to gold, especially when it comes to their talent. Not sure tho, as it's still a new series. I personally hope Silver have more than what's initially seen.