First time highlights. Advice needed

Hi everyone. I am not really into highlights but got bored of my natural colour so thought i'd give them a try. My hair is dark brown and i started home dying it regularly since i discovered few greys which have multiplied since. I always stick to my natural colour as lighter colours don't normally work on my very dark hair. I then came across l'oreal highlights glam which looked fab and easy to use so i thought why not. I watched video tutorials by people with the same hair shade as me and the result looked amazing on them so i felt encouraged. I bought the kit yesterday and found it very easy to apply. It said to leave it for 45 mins but i washed it off after 25 mins and thank god i did! The highlights were the brassiest i have ever seen my hair looked scary and felt awful to thr touch. There was no way i would go to work like this tomorrow. Luckily i have also bought a permanent colour in a light ash brown in case things went wrong and it came very handy indeed. I washed it off after 15 mins instead of the recommended 25 hoping that my hair wouldn't go very dark and it didn't. The result was a gorgeous rich brown with defined deep toffee highlights which is what i wanted it in the first place. My question is how do i maintain this new colour? It will fade after 3 or 4 washes and i have no idea what colour i will end up with. Will the brassy highlights return? Do i use the same ash brown again to dye over? I really like this colour but i don't want to highlight my hair again and don't want to go darker either! Grateful for advice. Thank you