Handle images with transparency

* Prevent Color from receiving too many arguments

  I noticed this passing 4 values for a GIF with transparency. ImageMagick
 outputs in the following format in that case:

      25,2: (228,229,237,  0)  #E4E5ED00  srgba(228,229,237,0)

  Which causes an extra argument to be passed.
* Fix rows with no colors (transparent rows) causing issues with
 clustering.