I cannot see that as a very good way to try and recover a tiny amount of silver plate, by dissolving loads of base metals in loads of acids, creating very large waste steam to deal with for almost no return or gain.
The solution is saturated with metal sulfate salts some of which will give color to the solution as ions as long as those metal ions are in the solution they will color the solution.
Silver sulfate does not have a high solubility, so much of your silver may not be in solution, what is can be tested with a little salt or HCl.
Silver corrodes as a sulfide giving a blackish oxide color to silver, that is sulfides but it may be you are seeing something similar with the silver sulfates, I am not sure if this could have something to do with what you are seeing, also silver salts will react with light (normally after the acid or oxidizers are washed from the salts).
Metals like iron can color sulfates black or sulfuric acid black. Copper will give blue.
With plated scrap metal you can be dealing with almost any metal or its alloys.
I would be surprised if your solution was not colored at this point.