It's mostly in the lineart. (If you're using Photoshop) Duplicate your base lineart and Gaussian Blur the copy (I do it at 1.0-3.0), then lower the opacity of the copy til the desired effect if reached.

As for the finished/colored version, blur the end result and fade (ctrl+shift+F) it to the 'Lighten' layer option. That's how I normally do it.