For multi-layer contour graph, check Common Plot Properties will use union Z scale range.
For multi-layer contour graph, check Common colormap and then rescale colormap in any layer will find union range to cover all Z.
The Common colormap checkbox is added under Layers tab of page level in Plot Details dialog. Select Plot Properties should disable and select this checkbox. Select it without Plot properties will share colormap related settings(scale, fill, line, label) only.
We add Colormap to dropdown of Common Display button in page level MT, under Plot Properties.