But if, for example, the author made it clear that the other kids were all wearing the same color shirt and the main character was not, you might infer something about their relationship from that--like maybe that the other kids all went to the same school, but the lead character went to a different school.
Details do matter for reading comprehension, because every detail is chosen. The real world is full of superfluous detail... writing, especially good writing, is not. Every word on the page represents a choice by the author--a choice to put that detail in, and to use that particular word instead of a different one.
Personally I don't really appreciate any comprehension tests that can be answered somewhat easily by computers, which the "shirt color" thing could be. And tests that are hacked by simple human algorithms are boring. In elementary school I got really far ahead in extra credit reading packets by realizing I could skip the reading and just go straight to the questions, skim for the answer, repeat. Somehow I think that certain peers in high school never mastered this trick since they were complaining about lack of time on the ACT... The better test of comprehension was to read something and then write about it, but the writing section of the ACT was separated and made optional.
Just because every bit of writing is a choice, that doesn't mean the author him- or herself fully understood why they made that choice. We're all shaped by our own life experiences, our family, the time in which we grew up, etc. Often, it's easier for a third party to connect those dots. That's why coaching and therapy works, for instance.
That said, when we're talking about standardized tests, they tend to (and should be) sticking to pretty well-understood and obvious examples of this sort of thing.
And of course, sometimes they're just not well done. Standardized test writers face the same challenges as every other writer, and I think we can guess that the very best writers probably do not end up in that job...
Maybe that is reading comprehension and I am just bad at it, but to me shirt color was unimportant.