You can, though it might take a while depending on how many units you want to change; basically you go into any unit's _anim.txt file (whatever the latest version number subfolder is they appear in) and there will be four variations of most units' animations beneath each of their TechLogics (one per anim heading, like Idle, Attack, Walk, etc.). Each will correspond with a tech level. For example, Hoplites have this beneath their hoplite_anim.txt anim Idle section:

To make the change you want, you'd swap the text that says "Infantry G Hoplite Bronze" and "Infantry G Hoplite Iron" which are the names of their silver and golden armor textures, on their bodies - not heads - respectively, even though yeah it sounds like the opposite. This change alone will only affect their idle animation, so you have to do the same thing for each anim type in the file.

I won't get into the basics of making a new mod here, just don't save over the existing hoplite_anim.txt file. Know that you need to match the file structure in your own mod's named folder to the base game's structure in order to override the unit's texture when your mod is activated, including the version1.0 (or whatever number the original file came from) subfolder.

If I were you, before editing anything I'd download a mod you like that affects unit appearances, such as Nick3069's Human Units Visible Upgrades and use those files as a starting point instead of the base game's, since two mods cannot affect the same file - this way you can essentially merge your desired changes with another mod.

(Also I totally agree with the premise of your mod; I always hated that units turn into garishly ugly gold for their final upgrade which is clearly supposed to be bronze.)

Thanks for the reply, but unfortunately I'm not a moder so I'm not sure if I could do all that properly.
The only mod I've ever done was a very simple one where I changed the colors of the players thanks to a mod guide that showed how to do it yourself. I thought that changing the color of the armor would work the same way, but it seems that is not the case.

I suppose there's an easier way i should have thought of before, though still not as simple as changing a few RGB numbers: you could find the textures you want switched (/Age of Mythology/textures/) and swap the names on those files for your mod. It could have unintended interactions with other mods, but certainly nothing game-breaking.

Again though, you'd need to change like 2-6 filenames per-unit you're trying to effect, which would certainly add up.

Thank you, but I don't think I expressed myself well.
What I want is not to change the models, what I want to change are the colors of the models, that is, the armor of a campion in silver and gold for a heavy.

That's what this would do. Units' body models don't change with upgrades so changing the textures would just change the color of their armor.

Oh, I see, thanks for the correction. I'll see if I can change them.