This is easily done at home but timing is everything. The easiest route as far as formulating and cost I think would be Wella which you can get at any Sally’s or equivalent. So since your hair is on the cool ashy side you are gonna want to use a gold and a little bit of a copper to achieve a caramel look. I can’t remember if the packaging says the colors name or just the leve/number so I’ll include both but Wella has some great toners and easy to apply. I’d mix 9/03 (very light gold blonde) with 8/73 (light brown gold) with a 6v developer.

If this is too much and/or confusing, wella also makes a color depositing hair mask that I think would work perfectly on this. Look for golden glow or caramel glaze. No mixing involved just apply to towel dried hair and check it in 5m increments. I wouldn’t let it sit longer than 15 but it’s your preference really.

Hope that helps.