Highlights & Toner Shade

Brown hair with highlights
Photo: Mikhail Malyshev/Shutterstock
Q: Can I put a toner shade over my highlights as they have come out a bit too bright and I want to tone them down. It is a {brand omitted} tone and shade in rich, velvet brown. Please advise a.s.a.p.
A: As long as your hair hasn't been damaged by the highlighting process, you should be fine to add a toner (specifically a "deposit-only" color toner). However, the color you describe as what you intend to use may not be what you want to use.
Remember that hair color - unless it has a lifting action - will combine with the natural color and result in a color that is darker than expected. The deposit-only action of a toner requires that you use a shade that is only slightly darker than your highlights are currently, and that you make certain to mix the color with a developer that offers no lifting action.
You need to be aware that a "toner" is meant to be a deposit-only process, therefore it will only add color to the hair, and if the hair is lighter than the color intended by the "toner" the lighter hair will be darkened to that level.
The color you mention above - being a rich, velvet brown - will most likely simply darken your hair to a uniform color and will obliterate your highlights, covering them completely instead of subduing them as you want.
See also:
How to color hair
Highlights and highlighting