For the most part the robes of the monks depended on the dye that was available in the region. And then tradition just kept those different colors. And it also helps distinguish which sect/tradition/school of Buddhism the monastic follows.
The simplicity of wearing a basic robe partly symbolizes the vow they have taken to live a simple life. It is like their "uniform" in a way. A symbol of their non-status that they are no longer partake in the material aspects of society.
The material and dyes for their robes are usually donated by the laity.
The robe also symbolizes the monks connection to the Buddha and his willingness to follow in his footsteps.
Within some Tibetan Buddhist schools, If their sleeveless tunic is trimmed with yellow brocade or they are wearing yellow silk and satin as normal attire, they are probably eminent monks or considered living Buddhas. This link will help describe how the robes have changed over time.
Some, however, consider robes to be elitist and encourage pride as one "advances" within ones sect.
As for monks shaving their heads, it often symbolizes the renunciation of worldly things. It helps monks over-come vanity to embrace the simple life of a monk.
I hope this little research has helped a bit.
P.S.~Just wanted to let everyone know that the blog has just passed over 100,000 hits. The number doesn't mean as much to me, whereas, the readers that number represents is what is notable and humbling for this imperfect manifestation called James. Thank-you to everyone for your support, readership and comments.
I bow to the Buddha within you all. _/I\_
~Peace to all beings~