Getting Back to Natural Hair Colour After Dyeing?

I dyed my hair red a month ago (I used Lorvenn's 5.22 and 7.22 1:1 ratio) and a couple of days ago I dyed it dark brown (4.15 by Garnier's Olia brand). The red still shows through. I really want to go back to my natural hair colour, which is a medium to dark brown or at least a dark brown. Do I have to bleach it and redye it in order to go back to brown? If I just let it grow out will the colour wash away a bit? I really don't know what to do.