Nikon Z9 Mirrorless Camera - Manual (Page 610)

610 A Custom Settings: Fine-Tuning Camera Settings
When focus peaking is enabled in manual focus mode, objects that are
in focus are indicated by colored outlines in the display. You can choose
the color.
Focus Peaking Display
Select [ ON ] to enable focus peaking.
Focus Peaking Sensitivity
Choose focus peaking sensitivity from [ 3 (high sensitivity) ], [ 2
(standard) ], and [ 1 (low sensitivity) ]. The higher the value, the greater
the depth that will be shown as being in focus.
Focus Peaking Highlight Color
Choose the highlight color.
a13: Focus Peaking
G button
U A Custom Settings menu