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Specifying Colors

Matplotlib recognizes the following formats to specify a color:

  • an RGB or RGBA tuple of float values in [0, 1] (e.g., (0.1, 0.2, 0.5) or (0.1, 0.2, 0.5, 0.3)). RGBA is short for Red, Green, Blue, Alpha;
  • a hex RGB or RGBA string (e.g., '#0F0F0F' or '#0F0F0F0F');
  • a string representation of a float value in [0, 1] inclusive for gray level (e.g., '0.5');
  • one of {'b', 'g', 'r', 'c', 'm', 'y', 'k', 'w'};
  • a X11/CSS4 color name;
  • a name from the xkcd color survey; prefixed with 'xkcd:' (e.g., 'xkcd:sky blue');
  • one of {'tab:blue', 'tab:orange', 'tab:green', 'tab:red', 'tab:purple', 'tab:brown', 'tab:pink', 'tab:gray', 'tab:olive', 'tab:cyan'} which are the Tableau Colors from the ‘T10’ categorical palette (which is the default color cycle);
  • a “CN” color spec, i.e. 'C' followed by a single digit, which is an index into the default property cycle (matplotlib.rcParams['axes.prop_cycle']); the indexing occurs at artist creation time and defaults to black if the cycle does not include color.

“Red”, “Green” and “Blue”, are the intensities of those colors, the combination of which span the colorspace.

How “Alpha” behaves depends on the zorder of the Artist. Higher zorder Artists are drawn on top of lower Artists, and “Alpha” determines whether the lower artist is covered by the higher. If the old RGB of a pixel is RGBold and the RGB of the pixel of the Artist being added is RGBnew with Alpha alpha, then the RGB of the pixel is updated to: RGB = RGBOld * (1 - Alpha) + RGBnew * Alpha. Alpha of 1 means the old color is completely covered by the new Artist, Alpha of 0 means that pixel of the Artist is transparent.

All string specifications of color, other than “CN”, are case-insensitive.

“CN” color selection

“CN” colors are converted to RGBA as soon as the artist is created. For example,

import numpy as np
import matplotlib.pyplot as plt
import matplotlib as mpl

th = np.linspace(0, 2*np.pi, 128)

def demo(sty):
    fig, ax = plt.subplots(figsize=(3, 3))

    ax.set_title('style: {!r}'.format(sty), color='C0')

    ax.plot(th, np.cos(th), 'C1', label='C1')
    ax.plot(th, np.sin(th), 'C2', label='C2')

  • ../../_images/sphx_glr_colors_001.png
  • ../../_images/sphx_glr_colors_002.png

will use the first color for the title and then plot using the second and third colors of each style’s mpl.rcParams['axes.prop_cycle'].

xkcd v X11/CSS4

The xkcd colors are derived from a user survey conducted by the webcomic xkcd. Details of the survey are available on the xkcd blog.

Out of 148 colors in the CSS color list, there are 95 name collisions between the X11/CSS4 names and the xkcd names, all but 3 of which have different hex values. For example 'blue' maps to '#0000FF' where as 'xkcd:blue' maps to '#0343DF'. Due to these name collisions all of the xkcd colors have 'xkcd:' prefixed. As noted in the blog post, while it might be interesting to re-define the X11/CSS4 names based on such a survey, we do not do so unilaterally.

The name collisions are shown in the table below; the color names where the hex values agree are shown in bold.

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)):
    weight = None
    cn = mcd.CSS4_COLORS[n]
    xkcd = mcd.XKCD_COLORS["xkcd:" + n].upper()
    if cn == xkcd:
        weight = 'bold'

    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.axhline(j, color='k')

ax.text(.5, j + 1.5, 'X11', ha='center', va='center')
ax.text(1.5, j + 1.5, 'xkcd', ha='center', va='center')
ax.set_xlim(0, 3)
ax.set_ylim(0, j + 2)

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