I don't wear a Toplace system, so I didn't feel eligible to reply.
But regarding brassy tones, I recently tried a product called Aufero because my hair can fade to a bright yellow in the sun.
Its a colour conditioner that restores original colour, as well as removing brassy red & yellow tones and was only available to buy through salons as its for professional use on replacement hair.

I was so pleased with the results, I contacted the company and became a distributor so that individuals could then purchase it.

Unlike Back2Natural, you don't have to use it regularly to maintain your colour.
The Aufero is more of a treatment that you use as-and-when you need to.
It works on natural and replacement hair that has faded.
Below, my natural hair went yellow in the sun, but my replacement hair remained ash.
It comes in 5 shades and I applied Dark Ash Blonde by combing it through my natural hair and that one application has lasted 2 months.
Right now, my system hair has started to fade but my natural side hair has remained ash, so its time to do an application on top to keep it all the same.

Before discovering this product, I was dying my hair every 5 minutes and the results were variable because the replacement hair absorbed the dye faster than my natural hair and the act of dying it just made it fade more quickly.
I found its much easier, more consistent and better for the hair to use the Aufero