code: def thresholdForIdentity(identity, colors):
      
      for threshold, _ in colors:
          if identity >= threshold:
              return threshold
      raise ValueError('This should never happen! Last threshold is not 0.0?')
docs: Get the best identity threshold for a specific identity value.
  
      @param identity: A C{float} nucleotide identity.
      @param colors: A C{list} of (threshold, color) tuples, where threshold is a
          C{float} and color is a C{str} to be used as a cell background. This
          is as returned by C{parseColors}.
      @return: The first C{float} threshold that the given identity is at least
          as big as.