Here's the function applied to the surface:
Plot3D[x + y, {x, -2, 2}, {y, -2, 2}, ColorFunction -> (ColorData["Rainbow",
To look in the upper right corner, with the same color function and scaling, I set ColorFunctionScaling -> False and manually scaled the color function to match the (global) minimum to zero and the maximum to unity using Rescale
Plot3D[x + y, {x, 1, 2}, {y, 1, 2}, ColorFunctionScaling -> False, ColorFunction -> (ColorData["Rainbow", Rescale[

source share