My advice would be to not use a box dye. Go to Sally’s and dependant on your colour buy a bottle of peroxide - the 10 volume if you want to maintain a colour, 20 volume for a lift (maybe 2/3 shades lighter than your roots).
Mix that with a toner 1 which is a tube of colour in a bowl, consistency should be like custard (1/2 a tube for shoulder length hair).
Apply to coloured ends first for 15/25 minutes then the virgin roots for a further 15/20 minutes.
Wash off with shampoo and double condition.
Your colour will last longer than a box dye.
It’s more expensive initially, but you should get 2/3 applications from your toner and the peroxide lasts for around 5/6 overall applications in my experience.