Dear brother Mathai,
The correct word is not 'Mumsona', its 'Musnapsa'.(Shirovasthram)
Each of the vestments a Bishop or a priest wear has some relation to Bible as well meaning. The Musnapsa that a Bishop wear on the head has 13 crosses which reflects Jesus Christ and his 12 Apostles. It also resembles the painful crown that laid by jews on Jesus's head during crucification. In earlier days, it was used to cover entire face of monks so that they dont see the material world. But later on when the thinking to challenge this world has been the mission so that Bishops can lead the people in righteous way,they began to use this without covering the face, instead they put into back. It also symbolise the Crown that one will be awarded when the Final Judgement comes.
About priests wearing other dresses, in western countries, Formal dress like Suite is weared by priests 'cause of wheather conditions. Just a white long dress (loha or vella kuppayam) is not enough to escape from extreme cold conditions that prevails in most of the western countries. But while dressing, they use a neck tie which differentiates them from others. Those familiar with these living conditins would notice that.
About the Bishop wearing different colours (red and black) has also some situational and authoraterian difference. In Western Culture, only the ArchBishop in his diocese wear the black dress with a Red Belt which shows his Authority. Other Bishops wear black dress (black Loha). Their dress will always has Red colour in thier vestments (either inside of Black loha or outside as seperate). These due to climatic conditions as well. This red robe has reference to the Red Robe put on Jesus Christ in front of Pontious Pilate. They (Jews) teased him while and like that, Bishops who also represents the servants of God also wears this red robe. Black Robe is refered to as penitance and mourning.
Feel free to forward any questions, will try to answer all. Hope that those well known on these topic will correct me if am wrong.
thanks and prayers,
Eldose T J