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Many thanks, Ranier. Expressing the color percentages as decimal fractions worked. TEXstudio now draws the rectangle with the desired vermillion color.

% Define custom color.
% Vermillion values at: https://www.colorhexa.com/e34234
% NOTE XCOLOR package CMYK colors must be defined as percentages ...

Can TeX specify a line color in CMYK?
Steps to Reproduce
1. Declare {xcolor} package with [cmyk] color mode option.
2. Define custom color: vermillion
3. Draw line for rectangle with custom color.
BUG: Line color is black, not custom color.
% File name: TitlePageBorder.tex
% LaTeX version ...

Dear Mr. Kottwitz:

Many thanks for your suggestions!

My question should have explained that my goal is facing pages: left columns on left pages that face right columns on right pages. You recommended removing the /switchcolumn commands. That worked.

Footnote: The book is a translated novel. It ...

I am attempting to create a critical edition with parallel texts in two languages. The paracol package seems to create all the left pages first with facing blank right pages, then all the right pages with facing blank left pages. The Contents page is followed by 3 blank pages with "Contents" in the ...