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On two identical rectangles I use a swatch as fill color. The swatch is C=100, M=0, Y=0, K=0. On one rectangle I've set the Tint for the swatch to 10%, on the Other I've set the opacity to 10%. The result is not the same color on a white background. I'm not sure why this is.
I do understand one makes the rectangle transparent and the other does not, but on a white background I had not expected the difference because the Tint itself would also be calculated based on 0% being whit.