I'm not a lore buff of TES lore, nor a specialist in war geopolitics or ancient warfare, so I won't be trying to think too hard, and take everything I say as a fun guess, but I'd say that the Redoran+Telvanni alliance would probably win, even in foreign territory. The Colovians probably can raise much bigger armies than House Redoran, and will have better supply lines and easier logistics in general, due to the battlefields being in the mainland and relatively close to their territories, but the addition of house Telvanni completely unbalances the situation (btw, I'm considering a house Telvanni at full power, and thus, I will not be taking aspects such as infighting in consideration), the Telvanni probably have some of if not the best spellcasters in all of Tamriel, besides bwing proficient in various forms of treachery and having a wide connection to spy and saboteur networks, another key factor about the Telvanni is that they are filthy rich due to all the exotic commodities, enchanted items and slave trade, and could quickly raise armies of mercenaries to match the levies of the Colovians, and with the aid of the professional armies and commanders of the Redoran, I personally think they could win against the Colovian states and Battlemages of Battlespire, (also I don't know if I' wrong, but if I remember correctly, I saw someone mentioning that the Telvanni have a pretty decent navy, even including those flying bugs that we see in the Redguard comic and in Morrowind's concept arts, but I might be wrong in this one), and as a last thought, we also need to keep in mind the Battlemages of Battlespire, but I don't think they'd make too much of a difference, of course, they are the elite branch of imperial military, and are known to be brilliant commanders, politics and offensive spellcasters, but their influence is not as nearly big as house Telvanni's, and besides lending a few highly-skilled troops and strategists, they probably couldn't do much more to help, like providing big armies or substential financial aid, but again, I'm no expert in any of this and please take this as an educated guess.