GYM bag constructed with quality hardware and stiching.
Or one of these maybe???
Rifle bag maybe you can order it in a different color?
Maybe one of these but they come on black only
AWS
If you want total low pro I think a gym bag with a pull out insert and a smaller pack or bandolier inside is the way to go. Any way you go I think you will have to 1) buy a general bag and modify to your needs or 2) have one constructed to your specs.
I have a gym bag I plan to use for the very purpose you describe but inside I plan on having two smaller bags. One being a backpack and one a smaller GYM bag. Using the outside pockets on the big main bag for storage of items like my pistol, mags, ammo, holster, cash, surefire, folding knife etc. Once I start a movement during a SHTF event I plan on arming myself CCW and if need be I can take out the backpack wear it and use the smaller bag to hide my KISS AR and MAGS. The back pack is were I plan on having my support gear like food, comms etc.
The pack within a pack concept I think will aid also in camoflauge not only in the aspect of hiding a big ass gym bag, but as you may be able to put a extra shirt and hat in it. Just make sure you take out the bottom cardboard insert of most GYM bags so you can compact it into the backpack.