Canon EOS-1D X Mark III Advanced User's Manual page 405

Setting MF peaking (outline emphasis)
In Live View shooting, edges of subjects in focus can be displayed in color to make focusing
easier. You can set the outline color and adjust the sensitivity (level) of edge detection.
1.
Select [
2.
Select [Peaking].
3.
Set the level and color.
: MF peaking settings].
Select [On].
Set it as necessary.
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