Color palette with python

Germain Salvato Vallverdu germain.vallverdu@univ-pau.fr

This notebook aims to present several ways to manage color palette with python, mainly for plot purpose.

Import some packages



In [1]:

    
import numpy as np
import matplotlib as mpl
import matplotlib.pyplot as plt
from IPython.display import HTML # intégration notebook
%matplotlib inline

Functions

Define convenient functions to plot a matplotlib colormap or a list of colors with matplotlib.

See this gist



In [2]:

    
def plot_cmap(cmap, ncolor=6):
    """ 
    A convenient function to plot colors of a matplotlib cmap
    
    Args:
        ncolor (int): number of color to show
        cmap: a cmap object or a matplotlib color name
    """
    
    if isinstance(cmap, str):
        try:
            cm = plt.get_cmap(cmap)
        except ValueError:
            print("WARNINGS :", cmap, " is not a known colormap")
            cm = plt.cm.gray
    else:
        cm = cmap
            
    with plt.rc_context(plt.rcParamsDefault):
        fig = plt.figure(figsize=(6, 1), frameon=False)
        ax = fig.add_subplot(111)
        ax.pcolor(np.linspace(1, ncolor, ncolor).reshape(1, ncolor), cmap=cm)
        ax.set_title(cm.name)
        xt = ax.set_xticks([])
        yt = ax.set_yticks([])
    return fig



In [3]:

    
def show_colors(colors):
    """
    Draw a square for each color contained in the colors list
    given in argument.
    """
    with plt.rc_context(plt.rcParamsDefault):
        fig = plt.figure(figsize=(6, 1), frameon=False)
        ax = fig.add_subplot(111)
        for x, color in enumerate(colors):
            ax.add_patch(
                mpl.patches.Rectangle(
                    (x, 0), 1, 1, facecolor=color
                )
            )
        ax.set_xlim((0, len(colors)))
        ax.set_ylim((0, 1))
        ax.set_xticks([])
        ax.set_yticks([])
        ax.set_aspect("equal")
    
    return fig

Color models

How to define a color ?

RGB : Red Green Blue, additive color model
HSL : Hue Saturation Light
CMYK- : Cyan Magenta Yellow Black, soustractive color model

RGB

Red Green Blue

an additive color model
set "the amount" of a each based color
each color is defined by un nombre between 0 and 255 (8 bits = 1 octet)

red (255, 0, 0)
green (0, 255, 0)
blue (0, 0, 255)

HTML color codes

HTML color codes are another way to define a RGB color using an hexadecimal numeral system.

HTML color starts with a # character
the folowing 6 characters defines the intensity of red, green and blue colors
each pair correspond to a number in hexadecimal numeral system from 0 to FF (255)

example : #2D85C9 <-> (48, 133, 201)

2D -> 48
85 -> 133
C9 -> 201

RGB and RGBA

extension of RGB model
a for alpha (canal alpha)
RGBA = RGB + opacity
opicty is defined between 0 (transparent) and 1 (opaque)

HSL

Hue, staturation, light

Closer to the human perception of the color than RGB model
Hue is define on a wheel from an angle between 0 and 360°
Saturation and light are defined between 0 et 100%

CYMK

Cyan Yellow Magenta Black

Based on a substractive color model
Used by printers

Several kinds of palette

qualitative palettes
sequential palettes
divergentes palettes

Qualitatives palettes

Used to distinguish several groups of data without order or relationship

Example



In [4]:

    
plot_cmap("Dark2", 4)









    Out[4]:



In [5]:

    
plot_cmap("Dark2", 4).savefig("img/qualitative.png", bbox_inches="tight")

Sequential palettes

Color scale
Use it if there is an order between the data

Example



In [6]:

    
plot_cmap("Blues", 8)









    Out[6]:



In [7]:

    
plot_cmap("Blues", 8).savefig("img/sequentielle.png", bbox_inches="tight")

Reverse order

In matplotlib all palettes suffixed by _r are reversed.



In [8]:

    
plot_cmap("Blues_r", 8)









    Out[8]:



In [9]:

    
plot_cmap("Blues_r", 8).savefig("img/sequentielle_r.png", bbox_inches="tight")

Divergent palettes

Usefull if there is a central value which play a specific role
values disposed around zero

Example



In [10]:

    
plot_cmap("coolwarm", 9)









    Out[10]:



In [11]:

    
plot_cmap("coolwarm", 9).savefig("img/divergente.png", bbox_inches="tight")

Build a custum color palette

some tools

With matplotlib

matplotlib colormaps

matplotlib colormaps are in matplotlib.cm module
colormap allows to map values on a color scale

Example colormap `summer`



In [12]:

    
plot_cmap(plt.cm.summer, 6)









    Out[12]:



In [13]:

    
plot_cmap(plt.cm.summer, 6).savefig("img/summer.png", bbox_inches="tight")

colormap returns a rgba color.



In [14]:

    
plt.cm.summer(X=42)









    Out[14]:





(0.16470588235294117, 0.58235294117647063, 0.40000000000000002, 1.0)

X is a float number or a list or an array
X must be between 0 and plt.cm.colormap.N
You can define opacity



In [15]:

    
print("Max val = ", plt.cm.summer.N)
palette = plt.cm.summer(X=[1, 50, 100, 200], alpha=.6)
print(palette)
show_colors(palette)









    



Max val =  256
[[ 0.00392157  0.50196078  0.4         0.6       ]
 [ 0.19607843  0.59803922  0.4         0.6       ]
 [ 0.39215686  0.69607843  0.4         0.6       ]
 [ 0.78431373  0.89215686  0.4         0.6       ]]






    Out[15]:



In [16]:

    
show_colors(palette).savefig("img/mpl_palette1.png")

You can change interval values using Normalize



In [17]:

    
normalize = mpl.colors.Normalize(vmin=-5, vmax=5)
palette = plt.cm.summer(X=normalize([-4, -2, 0, 2, 4]), alpha=1)
print(palette)
show_colors(palette)









    



[[ 0.09803922  0.54901961  0.4         1.        ]
 [ 0.29803922  0.64901961  0.4         1.        ]
 [ 0.50196078  0.75098039  0.4         1.        ]
 [ 0.70196078  0.85098039  0.4         1.        ]
 [ 0.90196078  0.95098039  0.4         1.        ]]






    Out[17]:



In [18]:

    
show_colors(palette).savefig("img/mpl_palette2.png")

With colorlover



In [19]:

    
import colorlover as cl

Colorlover provides function to set up a color palette

import colorlover as cl

cl.colorsys : conversion between color models
cl.scales : color palette
cl.to_HTML : help function to show the palette

cl.scales has to be used following :

cl.scales["number"]["type"]["name"]

where

number is a number between 3 and 12 included
type is : div, seq or qual
name is the palette name

All palettes are not available for all combinations.

For example, this is divergent palettes with 4 colors. You have to use cl.to_html to get an html version and HTML() to ask the notebook to display the html code and show the palette.



In [20]:

    
HTML(cl.to_html(cl.scales["4"]["div"]))









    Out[20]:




BrBG
PiYG
RdYlBu
RdGy
RdBu
PuOr
PRGn
RdYlGn
Spectral

Divergent color palette PuOr with 4 colors :



In [21]:

    
cl.scales["4"]["div"]["PuOr"]









    Out[21]:





['rgb(230,97,1)', 'rgb(253,184,99)', 'rgb(178,171,210)', 'rgb(94,60,153)']

conversion in RGB triplets

In order to use this palette with matplotlib you have to convert it in true RGB triplets.



In [22]:

    
cl.to_numeric(cl.scales["4"]["div"]["PuOr"])









    Out[22]:





[(230.0, 97.0, 1.0),
 (253.0, 184.0, 99.0),
 (178.0, 171.0, 210.0),
 (94.0, 60.0, 153.0)]

With seaborn



In [24]:

    
import seaborn as sns

The documentation is really clear and provides a nice tutorial seaborn color palettes. The following juste provides simple exampl cases.

import seaborn as sns

seaborn provides several functions in order to build and show color palettes. For example, in order to show the current palette :



In [29]:

    
current_palette = sns.color_palette()
sns.palplot(current_palette)

A qualitative palette



In [26]:

    
sns.palplot(sns.color_palette("husl", 8))

A sequential palette



In [27]:

    
sns.palplot(sns.light_palette("violet", 4))

A divergente palette



In [28]:

    
sns.palplot(sns.diverging_palette(220, 20, n=5))

Enjoy !

Germain

Color palette with python

Germain Salvato Vallverdu germain.vallverdu@univ-pau.fr

Import some packages

Functions

Color models

How to define a color ?

RGB

Red Green Blue

HTML color codes

RGB and RGBA

HSL

Hue, staturation, light

CYMK

Cyan Yellow Magenta Black

Several kinds of palette

Qualitatives palettes

Example

Sequential palettes

Example

Reverse order

Divergent palettes

Example

Build a custum color palette

some tools

With matplotlib

Example colormap summer

With colorlover

conversion in RGB triplets

With seaborn

A qualitative palette

A sequential palette

A divergente palette

Enjoy !

Example colormap `summer`