I think the suit design itself, not just its age, is meant to reflect Hal being the old guard

I feel like if you're an art director or costume designer or whatever for the show, and you have to make a costume that represents an old veteran, it's natural to make something that looks old-fashioned itself (not just a regular hip costume that happens to be faded). To me the brown reminds me a bit of, like, medieval armor or something almost ancient.

Again, I'm not just saying the suit looks aged because Hal has been doing this for a while (everyone else has already said that); what I mean is that the brown color and bulky pattern may be meant to feel old-fashioned and clunky. So I personally don't think it'll light up green or get dusted off and look brighter, because I think it's meant to connote age in every aspect of its design.

Then if John Stewart gets a sleeker, more aesthetic, more bright-green costume? Well you have an instant visual difference and it's even more exciting to see John become our new Green Lantern.

And even though there are other costumes in the closet, I have a feeling the one we saw will be Hal's primary one (although I also think we'll barely see them in costumes because they're doing inconspicuous detective work in populated places most of the time).

To put it another way, I think Hal having a brown suit of armor as a costume serves the same function as him having a scruffy beard in the show.

I know everyone's arguing about the trailer, these are just some thoughts. Please don't yell at me