Hans,
You always need to use ColorFunctionScaling->False if you want to use absolute number values from the function.
This works:
Plot3D[x^2 + y^2 - 100, {x, 1, 10}, {y, 1, 10},
ColorFunction -> Function[{x, y, z}, If[z > 0, Hue[0.2], Hue[0.5]]],
ColorFunctionScaling -> False]
or
Plot3D[x^2 + y^2 - 100, {x, 1, 10}, {y, 1, 10},
ColorFunction -> Function[{x, y, z}, If[z > 0, Red, Blue]],
ColorFunctionScaling -> False]
Regards,
Neil