Winter cherry

Winter Cherry, used for dyes and changing mutagens to a different color, can be acquired at the green area at your estate after receiving it from the queen in the witcher 3: Blood and Wine expansion

In order to receive the estate you need to continue the story until you come to a part where the victims of the murders around Toussaint are summarized.

After arriving at your house/estate, you will be shown around the estate.

Next thing to do is do all renovations of your house at the butler in order to be able to plant stuff in the garden. Once done just meditate to skip the days. After which you'll be able to find the Winter Cherry near your house (facing left-front when you leave your house). In there is a small chapel where the Winter Cherry will grow.

-Macgrat