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Each ink's middle tone against the accent, as most readers see it and as Machado, Oliveira and Fernandes's model (2009) shows it for the three color blindnesses at full strength, measured as the distance between the two colors in Oklab. Under 0.10 they are hard to tell apart in a fine line (marked ☞); over 0.15 they are clear. Those two thresholds are this page's rule of thumb, not a standard.