Make the Duotone Effect Pick a Side
Two colors enter, luminance mediates, and your grayscale source leaves the press looking far more expensive than the meeting that approved it.
Quick answer
The duotone effect maps an image's dark-to-light structure between two colors. Ether-MOSH measures luminance, builds a dark hue and a bright complementary hue, then blends that mapping with the source using Mix. Start at 0.80 Mix and 0.85 Hue. This is live RGB color mapping, not a print-ready two-ink separation.
Open Ether-MOSH