The violet test using stannous chloride, before you added copperas, tell us you had gold in solution (which we already knew), but it also told us you did not over use on the oxidizer (or the tin chloride would have not reduced the gold to the purple color), it also proved to you your stannous chloride solution is good and working.
I am assuming here you just added copperas to a small test sample, and then retried the stannous chloride test on this same test solution, and did not see a reaction with the tin (violet color reaction).
Were the copperas (ferrous sulfate crystals) green (fresh) or were they brown (oxidized)?
If you had a few drops of gold in solution in a spot plate, and added fresh copperas crystal, you would see a brown ring (gold precipitate), this is also a test for gold in solution (similar to the stannous chloride test for gold), copperas reduces gold to metal, so trying the stannous test after using copperas on this solution would not show a violet reaction, as the gold is already reduced to metal by the iron in the copperas (ferrous sulfate crystal).
The green can come from iron (in the iron sulfate crystal) (copperas) and the chloride solutions, iron replaces the gold in solution.
If the copperas crystals are brown I would not try and precipitate gold with them, the copperas oxidizes in air and will not work well to precipitate gold.
You can make your own copperas fairly easily, and have a supply of beautiful green crystals to work with, I can explain how I make mine out of transformer laminates and dilute sulfuric acid, or a search you should also be able to find it.
I do not know for sure I understood your question or if you can understand how I explained this if not maybe someone can word it better.
