If anyone pops in here with the same issue - I figured it out.
Make a new file with the same dimensions but in the Advanced Settings put it to at least 18- or 32-bit gamma integer. If you've been squinting at it and think it's light enough as is, just select the shirt part and right click > Colors > brightness/contrast and crank that brightness up to a slightly uncomfortable level. If you've been doing it like I was, there's a good chance you might be saying to yourself, "Hey...that's a bit too bright and washed out." Nevertheless, it seemed to have worked. I had already zeroed the textures back out to black and white and pretty much eyeballed the midnight navy color and got pretty lucky. Isn't a perfect match, but when these dudes throw their off-brand shirts and pants in the Maytag every night it's a super-duper bet that not everything or anything at all will be a perfect, harmonious match. And that's a fact lol.
*edit: Before anyone notices, I was just testing stuff out. I know he's got a revolver with pistol magazine pouches. Need to work on that between the Glock and revolver kits.