In almost all places in matplotlib where a color can be specified by the user it can be provided as:

• (r, g, b) tuples
• (r, g, b, a) tuples
• hex string, ex #0F0F0F, or #0F0F0F0F (with alpha channel)
• float value between [0, 1] for gray level
• One of {'b', 'g', 'r', 'c', 'm', 'y', 'k', 'w'}
• valid CSS4/X11 color names
• valid name from the xkcd color survey These names are prefixed with 'xkcd:' (e.g., 'xkcd:sky blue') to prevent name clashes with the CSS4/X11 names.

All string specifications of color are case-insensitive.

Internally, mpl is moving to storing all colors as RGBA float quadruples.

There are 95 (out of 148 colors in the css color list) conflicts between the CSS4/X11 names and the xkcd names. Given that the former are the standard color names of the web, matplotlib should follow them. Thus, xkcd color names are prefixed with 'xkcd:', for example 'blue' maps to '#0000FF' where as 'xkcd:blue' maps to '#0343DF'.

.. plot::

   import matplotlib.pyplot as plt
   import matplotlib._color_data as mcd
   import matplotlib.patches as mpatch

   overlap = {name for name in mcd.CSS4_COLORS
              if "xkcd:" + name in mcd.XKCD_COLORS}

   fig = plt.figure(figsize=[4.8, 16])
   ax = fig.add_axes([0, 0, 1, 1])

   for j, n in enumerate(sorted(overlap, reverse=True)):
       cn = mcd.CSS4_COLORS[n]
       xkcd = mcd.XKCD_COLORS["xkcd:" + n].upper()
       if cn != xkcd:
           print(n, cn, xkcd)

       r1 = mpatch.Rectangle((0, j), 1, 1, color=cn)
       r2 = mpatch.Rectangle((1, j), 1, 1, color=xkcd)
       txt = ax.text(2, j+.5, '  ' + n, va='center', fontsize=10)
       ax.add_patch(r1)
       ax.add_patch(r2)
       ax.axhline(j, color='k')

   ax.text(.5, j + .1, 'X11', ha='center')
   ax.text(1.5, j + .1, 'XKCD', ha='center')
   ax.set_xlim(0, 3)
   ax.set_ylim(0, j + 1)
   ax.axis('off')