Sure, the process of changing that is pretty easy
All you need to do is access the mod file (there should be a way to open the folder with the Arma 3 launcher, it usually gives a warning but ignore that)
From there, find my mod, should be named something "@SPAR_65_Variant" and copy it into a different place; in there, navigate to "addons", this is where the SCM.pbo is, which contains all data; imagine this like a ZIP file, but you need a special program to open it (I use PBO manager); Inside is a file named "cfgWeapons.hpp"; open that (not sure if it works in the PBO screen, alternatively copy it to your desktop, edit, and then replace it inside the PBO) and inside is a list of all guns added; for each gun, theres a line with magazines[]={"30Rnd_65x39_caseless_black_mag"};
Replace the "30Rnd_..." with "SCM_25Rnd_65x39_SPAR_Mag_Red" (change the color at the end to what you like out of yellow, green white and red). Do this with all the guns there or only the ones you want and replace the file in the PBO. It might give some error about signatures but since youll be using this for private use, it doesnt matter. lastly, change the name of the mod folder (the one with the @ in front) so you can differentiate it from the steam one. Lastly, in the Arma launcher, you can click "add local mod" somewhere in the mod menu, where you have to choose the folder with the @ in front. Thats it! hope it works for you, tried to explain it as simple as possible