You should think about fractals as (invisible) clouds in the TG space; whites do something, blacks do nothing, negative 'blacks' also do something, especially if unclamped. So modulate those unseen fractals to patterns you need (stretching, adding, subtracting, mixing, masking...), and apply them to a terrain (after the firstly modified terrain has been calculated).
Color adjust does a hard clamp, if you use a (blue node) smooth step scalar, you can make a smooth step of your greys.
It's not always easy to see what you're doing as such, but a test surface layer can give you an idea if you plug the texture in, or even a big (vertical) card object if you texture that with your mix of greys).