We have the bellow plot:

Plot[Sin[2 x]^2 Cos[1/2 x]^2, {x, 0, 2 \[Pi]},
PlotStyle -> {{Thickness[0.008], AbsoluteDashing[{15, 4, 4, 5}]}}]


The desired case is to have a plot that the part of the plot near the horizontal line be Darker than higher ones (which are lighter) (as gradient). However I saw some examples for gradient plots in SE but they are for multi-plots not for just one plot.

Plot[Sin[2 x]^2 Cos[1/2 x]^2, {x, 0, 2 \[Pi]},
ColorFunction -> (Blend[{Blue, Yellow}, #2] &),
ColorFunctionScaling -> False,
GridLines -> Automatic,
PlotStyle -> {{Thickness[0.008], AbsoluteDashing[{15, 4, 4, 5}]}}]


Some other possibilities:

Plot[Sin[2 x]^2 Cos[1/2 x]^2, {x, 0, 2 \[Pi]},
ColorFunction -> Function[{x, y}, If[y > 0.2, Red, Black]],
ColorFunctionScaling -> False,
GridLines -> Automatic,
PlotStyle -> {{Thickness[0.008], AbsoluteDashing[{15, 4, 4, 5}]}}]


Plot[Sin[2 x]^2 Cos[1/2 x]^2, {x, 0, 2 \[Pi]},
ColorFunction -> Function[{x, y}, GrayLevel@y],
ColorFunctionScaling -> False,
GridLines -> Automatic,
PlotStyle -> {{Thickness[0.008], AbsoluteDashing[{15, 4, 4, 5}]}}]


Plot[Sin[2 x]^2 Cos[1/2 x]^2, {x, 0, 2 \[Pi]},
ColorFunction -> "DarkRainbow",
ColorFunctionScaling -> False,
PlotStyle -> {{Thickness[0.008], AbsoluteDashing[{15, 4, 4, 5}]}}]


Showing how to mess with the internal Graphics structure:

p = Plot[Sin[2 x]^2 Cos[1/2 x]^2, {x, 0, 2 π},  PlotStyle -> {{Thickness[0.008],
AbsoluteDashing[{15, 4, 4, 5}]}}];
l = Cases[p, Line[a___] :> a, Infinity];
f[x_] := Rescale[x, range, {0, 1}]
range = {Min@#, Max@#} &@l[[1, All, 2]];
p /. Line[___] :>  Line[l[[1]], VertexColors ->
(Blend[{Blue, Yellow}, #] & /@ (f /@ l[[1, All, 2]]))]