I haven't tried it yet but I used Sun In in my hair a few days ago and it has some brass. Last night after I finally washed out the Sun In I decided to put just 4 drops of Gentian Violet in my almost full bottle of green VO5 conditioner. I'll try it tonight and let you know what happens. It did turn the whole bottle blue, and stained the side of the container where it was dripped in regardless of the amount of shaking.
If you have never used Gentian Violet before, it will stain every single thing it comes in contact with so be careful! I got mine for a skin condition and it stained my skin purple for about a week after I stopped use, and it also stained clothing it rubbed off on. However it did eventually wash out so even if it turns my hair purple I won't worry. I hope. lol