I dont cast in brass but regularly use bronze. Since both are copper
alloys this might be why your metal is turning.
The pickle will turn yellow bronze alloys reddish if there is any
iron /steel in the solution. It chemically activates the copper in
the metal making it a surface barrier. Your pickle might be
contaminated from some iron. I keep a couple pickling mini vats
going. One with materials that dont matter, one for when I want that
effect. Some specifically that are iron free, including tongs or a
sieve/strainer for retrieving pieces.
Hard pickle will remove the copoer tint. Pickle with hydrogen
peroxide added to the solution.
Another silversmith friend referred to it as poor man’s plating.