If your reason for not wanting camo is that you want to be obvious (maybe to other hunters so you don't get shot) then you can always flash a backpack with reflective material, or use a hi vis pack cover such as those recommended for cyclists.
Home-modding a rucksack to incorporate the barrel ties and stock pocket probably wouldn't take you too long if you wanted to turn your favourite pack into one able to carry a gun.
If it is purely for fashion reasons, then you are at the mercy of the manufacturers, who will make those they think will sell - and if camo packs are the big sellers then that is what you will find.