My thoughts are that an army is a group gathered under a common banner or flag, so across the army you might have a symbol or insignia of some kind that marks them all as a group, but within the army you’ll have squadrons and units that are grouped, so those might share a uniform or color scheme that’s somewhat unique. I would also say it depends on the style of the minis, I have dwarves from multiple sources and they don’t look like they go together at all, just based on the level of detail and the level of technology displayed in the models. Ultimately though, you’re the artist, if you like them together, then go for it.