https://graphcoloringmethods.com/

                 Graph Coloring Methods
                   Daniel W. Cranston
  This is a graduate textbook about how to color
  graphs.  Each chapter studies a single method,
  and presents numerous examples applying that
  method, generally in order of increasing
  difficulty. The book is designed to be
  suitable for a topics course in graph
  coloring, as well as self-study. The chapters
  are listed below.

   1.  Greedy Coloring
   2.  Gadgets: Constructions for Lower Bounds
   3.  Recoloring
   4.  Vertex Identification: Coloring Planar
      Graphs
   5.  The Kernel Method
   6.  Deletion and Contraction: Nowhere-Zero
      Flows
   7.  Rosenfeld Counting
   8.  The Combinatorial Nullstellensatz                  [GCM-cover]
   9.  The Activation Strategy                    
  10.  The Vertex Shuffle
  11.  Precoloring Extension
  12.  The Potential Method
  13.  The Rest of the Story

  This book is (and always will be) freely
  available for download.  It is released under
  a Creative Commons
  Attribution-NonCommercial-NoDerivatives 4.0
  International License.  License details are
  available at https://creativecommons.org/
  licenses/by-nc-nd/4.0.

  If you would like a paperback version, you can
  purchase one on Amazon. In the United States,
  it costs $24. It is also available in other
  markets: US  CA  UK  FR  IT  ES  DE  AU  NL  
  PL  SE  JP