#There is a complete dictionary in colors.py in the matplotlib directory.
import matplotlibfor name, hex in matplotlib.colors.cnames.iteritems(): print(name, hex)

This is the complete dictionary:
cnames = {'aliceblue': '#F0F8FF','antiquewhite': '#FAEBD7','aqua': '#00FFFF','aquamarine': '#7FFFD4','azure': '#F0FFFF','beige': '#F5F5DC','bisque': '#FFE4C4','black': '#000000','blanchedalmond': '#FFEBCD','blue': '#0000FF','blueviolet': '#8A2BE2','brown': '#A52A2A','burlywood': '#DEB887','cadetblue': '#5F9EA0','chartreuse': '#7FFF00','chocolate': '#D2691E','coral': '#FF7F50','cornflowerblue': '#6495ED','cornsilk': '#FFF8DC','crimson': '#DC143C','cyan': '#00FFFF','darkblue': '#00008B','darkcyan': '#008B8B','darkgoldenrod': '#B8860B','darkgray': '#A9A9A9','darkgreen': '#006400','darkkhaki': '#BDB76B','darkmagenta': '#8B008B','darkolivegreen': '#556B2F','darkorange': '#FF8C00','darkorchid': '#9932CC','darkred': '#8B0000','darksalmon': '#E9967A','darkseagreen': '#8FBC8F','darkslateblue': '#483D8B','darkslategray': '#2F4F4F','darkturquoise': '#00CED1','darkviolet': '#9400D3','deeppink': '#FF1493','deepskyblue': '#00BFFF','dimgray': '#696969','dodgerblue': '#1E90FF','firebrick': '#B22222','floralwhite': '#FFFAF0','forestgreen': '#228B22','fuchsia': '#FF00FF','gainsboro': '#DCDCDC','ghostwhite': '#F8F8FF','gold': '#FFD700','goldenrod': '#DAA520','gray': '#808080','green': '#008000','greenyellow': '#ADFF2F','honeydew': '#F0FFF0','hotpink': '#FF69B4','indianred': '#CD5C5C','indigo': '#4B0082','ivory': '#FFFFF0','khaki': '#F0E68C','lavender': '#E6E6FA','lavenderblush': '#FFF0F5','lawngreen': '#7CFC00','lemonchiffon': '#FFFACD','lightblue': '#ADD8E6','lightcoral': '#F08080','lightcyan': '#E0FFFF','lightgoldenrodyellow': '#FAFAD2','lightgreen': '#90EE90','lightgray': '#D3D3D3','lightpink': '#FFB6C1','lightsalmon': '#FFA07A','lightseagreen': '#20B2AA','lightskyblue': '#87CEFA','lightslategray': '#778899','lightsteelblue': '#B0C4DE','lightyellow': '#FFFFE0','lime': '#00FF00','limegreen': '#32CD32','linen': '#FAF0E6','magenta': '#FF00FF','maroon': '#800000','mediumaquamarine': '#66CDAA','mediumblue': '#0000CD','mediumorchid': '#BA55D3','mediumpurple': '#9370DB','mediumseagreen': '#3CB371','mediumslateblue': '#7B68EE','mediumspringgreen': '#00FA9A','mediumturquoise': '#48D1CC','mediumvioletred': '#C71585','midnightblue': '#191970','mintcream': '#F5FFFA','mistyrose': '#FFE4E1','moccasin': '#FFE4B5','navajowhite': '#FFDEAD','navy': '#000080','oldlace': '#FDF5E6','olive': '#808000','olivedrab': '#6B8E23','orange': '#FFA500','orangered': '#FF4500','orchid': '#DA70D6','palegoldenrod': '#EEE8AA','palegreen': '#98FB98','paleturquoise': '#AFEEEE','palevioletred': '#DB7093','papayawhip': '#FFEFD5','peachpuff': '#FFDAB9','peru': '#CD853F','pink': '#FFC0CB','plum': '#DDA0DD','powderblue': '#B0E0E6','purple': '#800080','red': '#FF0000','rosybrown': '#BC8F8F','royalblue': '#4169E1','saddlebrown': '#8B4513','salmon': '#FA8072','sandybrown': '#FAA460','seagreen': '#2E8B57','seashell': '#FFF5EE','sienna': '#A0522D','silver': '#C0C0C0','skyblue': '#87CEEB','slateblue': '#6A5ACD','slategray': '#708090','snow': '#FFFAFA','springgreen': '#00FF7F','steelblue': '#4682B4','tan': '#D2B48C','teal': '#008080','thistle': '#D8BFD8','tomato': '#FF6347','turquoise': '#40E0D0','violet': '#EE82EE','wheat': '#F5DEB3','white': '#FFFFFF','whitesmoke': '#F5F5F5','yellow': '#FFFF00','yellowgreen': '#9ACD32'}

#You could plot them like this:
import matplotlib.pyplot as plt
import matplotlib.patches as patches
import matplotlib.colors as colors
import math
fig = plt.figure()
ax = fig.add_subplot(111)
ratio = 1.0 / 3.0
count = math.ceil(math.sqrt(len(colors.cnames)))
x_count = count * ratio
y_count = count / ratio
x = 0
y = 0
w = 1 / x_count
h = 1 / y_count
for c in colors.cnames: 
    pos = (x / x_count, y / y_count) 
    ax.add_patch(patches.Rectangle(pos, w, h, color=c)) 
    ax.annotate(c, xy=pos) 
    if y >= y_count-1: 
        x += 1 
        y = 0 
    else: 
        y += 1
    plt.show()