Corrects the formula used in matcher threshold - use red mean not delta

See https://en.wikipedia.org/wiki/Color_difference#:~:text=%2C-,where,-%3A for reference of terms in formula

Bug: 299062874
Test: Presubmit passing
Change-Id: I091941d4c18365ca588ca586d3d884f05be1c034
1 file changed