Explain your idea

TL;DR:

Using more than one revolver in order to field different "loadouts" of ammo is currently impractical in IE due to weaknesses of the speedloader. This could be solved by adding the ability to match speedloaders to revolvers by color-coding them.

Useful gameplay feature? Yes, this would make it much easier to use the revolver as both a tool and a weapon at the same time.
Matches IE's Style? Yes, labeling which bullets go in which gun to avoid any mishap between the healing gun and the hurting gun seems in-line for an engineer.
Can be visually interesting? Not on it's own, but can contribute to the techno-gunslinger look of swapping between a variety of revolvers to handle any situation.
Viable to implement? I think so?

Proposal:
The revolver in IE is very unique firearm, as far as minecraft mods go, because of how you can customize the ammunition loaded into it in order to specialize it to specific situations. This can be as basic as loading one revolver with buckshot and another with armor piercing bullets to cover both short and long range with your big iron. With the versatility of phial bullets (as they can take any potion effect), there are even more possibilities (especially useful in multiplayer as a way to heal your friends or debuff your foes). However, the revolver has one major weakness: a long and complicated reload process. IE already offers two solutions to this problem: the extended magazine and the speedloader. These have their own issues, however. The extended magazine put simply, does not get along with speedloaders, and accidentally loading an extended mag revolver with a speedloader creates a bit of a mess. Speedloaders, meanwhile, lack any way to pick which one is loaded into a revolver other than their order in the inventory, so if you're trying to run multiple loadouts of bullets you'd probably spend as much time shuffling speedloaders around your inventory to get the right one in your gun as you would just loading the gun normally. Both of these issues, however, can be solved with my following proposal.

Feature Suggestion:
I suggest the ability to color-code revolvers and speedloaders, such that a colored revolver can only load a speedloader of the same color. This would solve both of the problems outlined above and generally make it much easier to use the revolver as both a weapon and a tool at the same time. (This solves the speedloader problem in a self-evident manner, and the extended magazine problem can be solved by labeling your extended magazine revolver a color that you don't have any speedloaders for)

I would suggest the color coding be achieved by crafting a revolver or a speedloader with a piece of dye in a crafting table, or undyed by placing it in a crafting table with nothing (alternatively the revolver could gain a new slot in it's engineer's workbench menu to add or remove a bit of dye while the speedloaders remain crafted). The dye on a revolver or speedloader could then be indicated by recoloring the square in the center of the cylinder UI that pops up when the revolver/speedloader is held/moused over in the inventory/opened. It could also be indicated on a speedloader's sprite by recoloring the center rod. Recoloring the revolver in a satisfactory manner may be difficult, so it can probably be neglected since the player can just code them with shaders and the UI will show the color anyway.

Implementation Viability:
I frankly lack the coding experience to determine this, but I do not think it would be super difficult to implement, As I think worse case scenario you could just brute-force it by copypasting a revolver item for each color that looks for it's matching colored speedloader when reloading? Or maybe just make it a tag on the revolver and tell the revolver to look for a speedloader with a matching tag when reloading? I may be misunderstanding how minecraft works. However, leather armor can be dyed without losing it's enchantments, so presumably this feature can be adapted to this suggestion.