I agree with Steve. It may depend on the values for which you will wish to provide the most visual contrast.
Let's say that you want to emphasize the contrast at 0.2. Then something like:
ColorFunction -> (Blend[{{0, GrayLevel[1]}, {0.15, GrayLevel[0.9]}, {0.25, GrayLevel[0.2]}, {1, GrayLevel[0]}}, #] &)
Compare:
Plot3D[Sin[x y], {x, 0, 2 Pi}, {y, 0, 2 Pi}, ColorFunction -> (GrayLevel[#3] &)]
Plot3D[Sin[x y], {x, 0, 2 Pi}, {y, 0, 2 Pi},
ColorFunction -> (
Blend[{{0, GrayLevel[1]}, {0.15, GrayLevel[0.9]}, {0.25, GrayLevel[0.2]}, {1, GrayLevel[0]}}, #3] &
)
]
And, you could do something similar by using an interpolation scheme for a ColorData scheme.
