cf[x_]cf1[x_] := Hue[2/3 x] cf2[x_] := Hue[2/3 - 2/3 x] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> cfcf1, PlotLegends -> Automatic] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> cf2, PlotLegends -> Automatic] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> "Rainbow", PlotLegends -> Automatic] 
cf[x_] := Hue[2/3 x] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> cf, PlotLegends -> Automatic]
cf1[x_] := Hue[2/3 x] cf2[x_] := Hue[2/3 - 2/3 x] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> cf1, PlotLegends -> Automatic] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> cf2, PlotLegends -> Automatic] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> "Rainbow", PlotLegends -> Automatic]
cf[x_] := Hue[2/3 x] Plot3D[{3.5 Log[1.7*x^3 + 0.8*Sin[y] + 3]}, {x, 1, 100}, {y, 10^-5, 1}, ColorFunction -> cf, PlotLegends -> Automatic]