Edit
Clear[plot, pts];
plot =
Plot[{Sqrt[(b^2 - 1)/b^2], -Sqrt[(b^2 - 1)/b^2]}, {b, -4, 4},
PlotPoints -> 50, MaxRecursion -> 2];
pts = Cases[plot, Line[pts_] :> pts, Infinity];
(*Graphics[{Polygon[Join[pts[[1]],pts[[2]]]],Polygon[Join[pts[[3]],\
pts[[4]]]]}]*)
Graphics[{EdgeForm[Blue], FaceForm[Green],
Polygon@Join[pts[[1]], Reverse@pts[[3]], Reverse@pts[[4]],
pts[[2]]]}, Axes -> True, AspectRatio -> 1/GoldenRatio]
Plot[{Sqrt[(b^2 - 1)/b^2], -Sqrt[(b^2 - 1)/b^2]}, {b, -4, 4},
MaxRecursion -> 10] /. {___, Line[pts1_],
Line[pts2_]} -> {FaceForm@Green, Polygon@Join[pts1, pts2]}
ContourPlot[
x^2 - y^2 == 1, {x, -2, 2}, {y, -2, 2}] /. {__, Line[pts1_],
Line[pts2_]} :> {FaceForm@Green, EdgeForm@Blue,
Polygon@Join[pts1, pts2]}
Original
Use RegionDifference
to get the complement of Filling -> {1 -> {2}
.
plot0 = Plot[{-Sqrt[(b^2 - 1)/b^2], Sqrt[(b^2 - 1)/b^2]}, {b, -4, 4},
AspectRatio -> 1, Filling -> {1 -> {2}}]
reg = RegionDifference[Rectangle[{-4, -1}, {4, 1}],
DiscretizeGraphics[plot0]];
plot = Plot[{Sqrt[(b^2 - 1)/b^2], -Sqrt[(b^2 - 1)/b^2]}, {b, -4, 4},
PlotStyle -> ColorData[97][1]];
Show[plot, Graphics[{Orange, reg}]]