>>> x = "Hello World!"
>>> x[2:]
'llo World!'
>>> x[:2]
'He'
>>> x[:-2]
'Hello Worl'
>>> x[-2:]
'd!'
>>> x[2:-2]
'llo Worl'

Python calls this concept "slicing" and it works on more than just strings. Take a look here for a comprehensive introduction.

slices to the rescue :)

def left(s, amount):
    return s[:amount]

def right(s, amount):
    return s[-amount:]

def mid(s, offset, amount):
    return s[offset:offset+amount]

You're looking for slicing:

>>> s = "Hello World!"
>>> print s[2:] # From the second (third) letter, print the whole string
llo World!
>>> print s[2:5] # Print from the second (third) letter to the fifth string
llo
>>> print s[-2:] # Print from right to left
d!
>>> print s[::2] # Print every second letter
HloWrd

So for your example:

>>> s = 'col555'
>>> print s[3:]
555