When I attempt to compile the MWE it rejects it regarding the optional arguments in background package.

@Calhistorian you have an outdated version of the package. Update the package, if possible; otherwise, I'll add an answer using the old syntax in a few hours ( bed time now).

@Calhistorian the code as it is now uses the outer margin (left margin on odd-numbered pages, right margin on even-numbered ones).

Please correct me (again) if I am wrong, but even in your MWE and the images displayed, the colored bar is in the inside margin. Right margin on odd and left on even.

@Calhistorian please see the numbering on the pages. The image seems to show the inner margin, but that's just due to the arrangement of pages in the viewer I used to obtain the image. If you check the numbering, you'll see that the outer margin is being used (in a normal text, odd-numbered pages go to the left, even-numbered ones, to the right: the image shows them swapped, but as I said that's just the image).

If I compile the MWE as you have it, Chapter 2 is an odd page with color in the north east corner. Your code states \ifodd then current page.north east. I of course don't presume to understand the code.

@Calhistorian precisely. Odd pages normally go to the left side on a regular book, so the first page of Chapter 2 has the mark on the outer margin; the second page of Chpater 2 is an even page, so it goes to the right in a book, and it has the mark on the outer margin. As I explained, my viewer presents facing pages as they appear in the image creating the illusion that the inner margin is being used, but this is not so. Remember, odd-numbered pages go to the left in a book (although in the image they appear to the right)

@Calhistorian (cont.) and even-numbered pages go to the right in a book (although in the image they appear to the left). In any case, I can provide the code to swap the marks, but then they will show up in the inner margins of a standard book.

Edited my original question to display the conflict I am having. I compiled your MWE to demonstrate. I suppose whatever the issue however, if you present the opposite code (that which switches the margin) we can stop the insanity. :).

@Calhistorian Hi! It's better to continue the discussion here.

:-)

I agree. Thank you for all your assistance. Now and (I do remember) in the past!

@Calhistorian You're welcome! Perhaps what confuses you is the position of the wider margin?

I think this is the path to discovery, yes. Am I missing something?

With the current settings of the document the wider margin is, in fact, the outer margin; this margin is, by default, so wide to accommodate the marginal notes.

In any case, I added to my answer the code necessary to place the marks in the inner margins, but the first code (the original one) is the one that uses the outer margins.

So when my document compiles and the even page has the wider margin on the left side, something is wrong? (like in the screenshot presented)

No, nothing is wrong. An even-numbered page has, by default, a wider margin on its left because marginal notes for even-numbered pages go to the left of the text (even-numbered pages appear always to the left when you have a book open wide).

Of course, you can change this default page-layout (using, for example the geometry package) if no marginal notes are going to be used.

Son of a ****. For some reason I had it stuck in my head that the even pages were on the right side of a book. I believe this is where it all began. Wow. My own stupidity. I assumed the wider margin was taking the book binding into account (gutter).

Yes, that's a very common misconception.

With the default settings, the wider margin is (wrongly) taken as the inner margin with the "binding space reserved", when in fact, it's the outer margin with space for the marginal notes, and the narrower margin (the inner one) is the one with the binding space.

Good to know. Thank you again. You may find in the future that your name is mentioned in the credits of a small local tutoring agency's publication. I am just saying.

Hehe! No problem. I'm glad I could help.

Bye!