That's a common mistake to make. The reason for this is `ColorFunctionScaling`. It remaps the range of values to run from 0 to 1 across the plot range. So basically, where `Sin[x]` is closest to -1, the filling is fully transparent, and where `Sin[x]` is closest to 1 it is fully opaque. The `Abs` here does not change anything.

First of all, you'll need to set `ColorFunctionScaling -> False`:

    Plot[Sin[x], {x, 0, 6.0}, PlotStyle -> Red, PlotRange -> All, 
     ColorFunctionScaling -> False, 
     ColorFunction -> Function[{x, y}, Hue[1, 1, 1, Abs[y]]], 
     Filling -> Axis]

![Scaling off][1]

But now it is white (or, rather, transparent) only very close to zero as opacity kicks in quite fast. If you must have transparency, you can, for example, replace the alpha argument with `1 - Abs[y]^3` or `Abs[y]^3` (depending, where you want red and where white):

![Different color function][2]

But if you ask me, the simplest way is just `ColorFunction -> (Blend[{Red, White}, Abs[#2]] &)`

    Plot[Sin[x], {x, 0, 6.0}, PlotStyle -> Red, PlotRange -> All, 
     ColorFunctionScaling -> False, 
     ColorFunction -> (Blend[{Red, White}, Abs[#2]] &), Filling -> Axis]

![Blend][3]

Note, that the filling here is white and not transparent. If transparency is necessary, you can try instead `(Blend[{RGBColor[1, 0, 0, 1], RGBColor[1, 1, 1, 0]}, Abs[#2]] &)`


  [1]: https://i.sstatic.net/UXXYZ.png
  [2]: https://i.sstatic.net/tOkuS.png
  [3]: https://i.sstatic.net/eTLPt.png