Vertical colorwork line

So I’m working on a patchwork afghan, and this is the next square I am going to do: https://www.stitchfiddle.com/en/c/sj7vl6-470re7/quickview

So for the border going around the square, I’m not sure what would be the best way to do this. Should I just do intarsia for that one stitch on either side on each row? Or would I be better off doing a duplicate stitch?

Awesome pattern! If it were me, I would double knit this. But, for a single-side work…either technique would work.

I’ve never had much luck with duplicate stitch. The background color always shows through the design. You might be better at it.

If you work it as intarsia, make sure you lock the yarn each time you change color. In case you haven’t worked intarsia before, here’s a video:

You would have to have separate strands of yarn for each section of color. I counted at least 4 of the background and 3 for the design including the vertical lines.

If you have any other questions, just ask.

For myself, I would go with the vertical lines in duplicate stitch. It will give a double thickness of yarn at the lines but I find that my single lines in intarsia tend to get lost in the flanking sts.
You might try each method on a swatch and see which works best for you.