This greatly interests me because I have never really thought about it. I understand that RoTK is a romanticisation during the Ming dynasty of the Three Kingdoms era. But what made Lu Bu famous, and where did the iconic armour come from, also, how realistic is it, if not, what kind of armour would he have worn if there's an answer to that?
Lu Bu's entire life's goal was to climb Han military ranks and eventually ascend into Han nobility after retirement and he worked really hard and sacrificed a lot for that goal: he was born in deep interior of what we know nowadays as the Mongolian Steppe and had to be adopted multiple times, change names and train harder than ever to become the best soldier there was to be able to rise to his current station--a commanding officer in forces guarding the Han's capital.
After Dong Zhou entered the capital, however, Dong Zhou's callous rule is making the force he serves under looked less and less like legitimate Han Imperial military and more and more like a private feudal/tribal army just happen to be occupying the capital. In simple terms, Dong was completely derailing Lu Bu's unblemished military career that he worked so hard and sacrificed so much to get, and Lu Bu was not about to lower himself to be a houscarl of some regional warlord when he was a general of the Imperial Army (if he's ok with being a glorified bodyguard of some powerful warlord or chieftain he would have stayed in the Mongol Steppe). The Romance claimed Lu Bu betrayed Dong for a lover to create drama, but I bet the real reason behind Lu Bu's betrayal and switch side to the Emperor and his ministers was the fact that his life ambition and aspiration is vanishing right before his eyes quick under Dong Zhou.
Lu Bu's much simpler aspiration in life (retire from Han Imperial military service honorably as a high ranking officer) was in direct conflict with Dong Zhou's lofty and treasonous ambition (what Dong Zhou and other warlords like Cao Cao, Yuan Shao, or Sun Jian wanted to achieve was the establishment of a new dynasty under their houses). Both the Romance and the Records pointed to the fact that Lu Bu was a much simpler man with a simple aspiration despite his martial prowess, and the fact that he was a much better subordinate officer and frontline soldier than as a independent warlord later on proves this point. It was a bad luck for him that his force that garrison the capital got simulated into Dong Zhou's Xiliang army when Dong Zhou first entered the capital under the order from the deceased Grand General of of Han, and it was also already too late when Lu Bu started to wise up to the fact that he was no longer a general in the Imperial Army but a defacto feudal vassal of a powerful regional warlord and was already in knee deep in some petty feudal conflicts that he believed to be beneath his station (This trailer shows very well that Lu Bu was not in his element and regrets his part in Dong Zhou's private feudal war with other warlords that he wants no part in if he had a choice).
I think Lu Bu is a very misunderstood figure and his constant switching sides is simply him trying to find the legitimate Imperial military which no longer exist after Dong Zhou's occupation of the Han capital to serve under. Lu Bu was simply too short sighted and can't see or refuse to see the future he doesn't want to see.
Zhao Yun was the same way initially: drifting from one lord to another in the quest to seek the true Imperial host, but he was lucky and found Liu Bei. Lu Bu, on the other hand, can't lower himself to serve Liu Bei as he was older than Liu Bei and was already well established and ranked as general in the Han's Imperial military when Liu Bei was still a landless drifter with a distant claim to the Imperial bloodline.
As for the armor he probably wore some armor bedecked in finery since everybody knew that the best way of getting on his good side was to give him gifts.
We know for a fact that he wore a red cloaks and a helmet with two long feathers. For the rest of his armor I cannot say.