Tooth Nerve

I Had Braces A Few Years Back And They've Left Me With A Dead Nerve In My Tooth. At First, This Tooth Went From Light Gray To A Darker Gray. And Now It Is Very Noticeable. It Doesn't Do Much For Confidence. I Had The Dead Nerve Removed A While Ago And Something In My Tooth Filled But Nothing Has Dimmed The Discolouration In My Tooth. What Treatments Are There On The NHS That Can Fix This Problem?