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.