Why is the inside of the Men-gane painted red? 

The obvious answer to this question is to reduce glare from the metal bars. This is only partially true, however. The actual answer is closely related to the previous bogu tidbit. As we know, a pale complexion on the battlefield was undesirable as it could be seen as fear on one’s face. Of course a samurai being scared on the battlefield was nothing less than a disgrace warranting seppuku, or worse.The component of samurai armor called the mempo was worn to protect the face. Mempo were made from rawhide or iron. With time and use the mempo would develop a greenish patina. When placed against the skin, this patina would radiate a greenish tint to the wearer’s face – not a good thing! To avoid this and to give a more vital appearance to the face of the samurai, armor makers would paint the inside of the mempo with red lacquer. Thus, when the mempo was worn, the red lacquer would radiate and give a vital glow to the complexion. This practice has continued to modern kendo armor, where bogu makers not only use red lacquer on the mengane to reduce glare, but also to give the kendoist a pink and lively facial radiance. (Ref: Kendo Nippon article)