Your colormap isn't changing, but by default MATLAB scales the colormap to the range of data in the plot. If your first surf plot, the Z data ranges from -2:2. So when that is the only plot, blue corresponds to -2 and yellow to 2.
[X,Y] = meshgrid(1:0.5:10,1:20);
Z = sin(X) + cos(Y);
surf(X,Y,Z);
colorbar
However, in your second surface, Znew ranges from 3 to 7. With both surfaces on the plot, blue still corresponds to -2, but now 7 corresponds to yellow. 2 now corresponds to turquoise.
Znew = Z+5;
figure
surf(X,Y,Z);
hold on
surf(X,Y,Znew);
hold off
colorbar
So before we can say how to fix the color scaling, could you explain what it is you want to happen?
