A friend on a server came up with the idea of this mod awhile back and I thought "hey that's a pretty good idea", so here it is.
Mod currently adds 15 variant colors of backpack, each with a different style and texture (it's not just a color change for each one). All backpacks work exactly the same as the default bag, if you are wearing multiple different colored bags the game will automatically use the bag in your first slot (or following slot if that one is empty).

As of writing this, pink and white leather currently lack any related backpack color, they will be added in another update. This is because all current backpack colors are based on the same palette as the books. Pink and white are now added as of 1.0.1. Mod also fixes some minor issues with the backpack model itself but it shouldn’t be noticeable.

All bag colors currently (Leather Bag):

• Brick Red
• Cherry Red
• Dark Beige
• Dark Gray
• Dark Green
• Dark Olive
• Gray
• Olive
• Orange
• Orange Brown
• Pink
• Purple
• Purple Orange
• Teal
• White

Planned Stuff:

• Possibly hunter backpack variants?
• More variants at the recommendation of others (aka if you want a new texture, recommend one!)