Redshifted from what? We need to know two of the following three to calculate the third: redshifted color, baseline color, and velocity.  The velocity is related to the difference between the redshifted color and the objects baseline color.  How do we know the baseline color of distant objects to know the amount of redshifting?
 A: Most of space is made up of Hydrogen. Hydrogen has distinct emission and absorption lines that correspond to electron orbital transitions. The 2 -> 3 line at 656 nm (so orange / reddish) corresponds to an electron going from a state with principal quantum number two to three. This will be the same everywhere (except perhaps near black holes) so there you have your baseline.
A: 
How do we know the baseline color of distant objects to know the
  amount of redshifting?


From Wiki:

To determine the redshift, one searches for features in the spectrum
  such as absorption lines, emission lines, or other variations in light
  intensity. If found, these features can be compared with known
  features in the spectrum of various chemical compounds found in
  experiments where that compound is located on earth. A very common
  atomic element in space is hydrogen. The spectrum of originally
  featureless light shone through hydrogen will show a signature
  spectrum specific to hydrogen that has features at regular intervals.
  If restricted to absorption lines it would look similar to the
  illustration (top right). If the same pattern of intervals is seen in
  an observed spectrum from a distant source but occurring at shifted
  wavelengths, it can be identified as hydrogen too. If the same
  spectral line is identified in both spectra but at different
  wavelengths then the redshift can be calculated

