Thank you for the reply and confirming what I was thinking! I really appreciate it.
This may be a convoluted workaround but I have found a theoretical way that I haven't tested but am sure of:
1. Make outline of lineart (either make the lineart , erase the insides and then copy to another layer, or trace the
lineart's outline on another layer). Make sure this layer is beneath the original lineart. Name it the silhouette
layer.
2. Make sure the lineart is closed and fill in with desired color with the Fill tool (twice to cover any gaps)
3. Now select the entire colored silhouette lineart and scale it to have it being slightly bigger than the original
lineart (which should be above the silhouette layer).
And that should work!
Edit: Just tested it and it works! Well, mostly. You will need to move around the scaling to taste and manually add in extra paint to areas as needed.