Getting a sample color in matplotlib

Is it possible to get an idea of what the color will appear in a plot by supplying the original RGB value (or other colormaps like RdYlBu) in python? Currently I have to really first convert a float number using cm.RdYlBu(x) and make a plot to see how the colors show up. But is there a quick way to check the color? Not necessarily in python, but any website that takes the value and returns the color according to the colormap would be fine.

Quick check in python console

You may use python itself to quickly show a figure with the respective color from the matplotlib colormap.

E.g. write a function

def c(x):
   col = plt.cm.RdYlBu(x)
   fig, ax = plt.subplots(figsize=(1,1))
   fig.set_facecolor(col)
   ax.axis("off")
   plt.show()

and then call it with the number of interest, e.g. c(0.4).

Example how that looks in a jupyter console:

A colormap viewer

Of course one could also write some color map viewer with matplotlib. For example:

import sys
import matplotlib.pyplot as plt
import numpy as np
import matplotlib.widgets

class CMViewer():
    def __init__(self, cmap, valinit=0.5):
        self.cmap = plt.get_cmap(cmap)
        col = self.cmap(valinit)
        self.fig, axes = plt.subplots(nrows=4, figsize=(5,3), num=cmap,
                                      gridspec_kw={"height_ratios" : [0.3,1,1,1]})
        self.fig.subplots_adjust(right=0.85)
        self.sliderax, self.cax, self.fax, self.lax = axes
        
        self.slider = matplotlib.widgets.Slider(self.sliderax, cmap, 0., 1., 
                                                valinit=valinit)
        self.slider.on_changed(self.update)
        self.slider.vline.set_color("k")
        
        x = np.linspace(0.,1.,256)
        self.cax.imshow(x[np.newaxis, :], cmap=cmap, aspect="auto")
        ta = dict(axis="both", bottom=0, left=0, labelbottom=0, labelleft=0)
        self.cax.tick_params(**ta)
        self.fax.tick_params(**ta)
        self.lax.tick_params(**ta)
        
        x = np.array([0,0,1,2,2])
        self.lax.imshow(x[np.newaxis, :], cmap="gray", aspect="auto", 
                        extent=[0,1,-0.2,1.2])
        self.lines = []
        for i in range(5):
            line = self.lax.plot([0,1],[i/4.,i/4.], alpha=0.2+i/5., color=col)[0]
            self.lines.append(line)
        
        self.update(0)
        plt.show()
        
    def fmtcol(self, col):
        return "{:.3f}  |  {:.3f}  |  {:.3f}".format(*col)

    def update(self, val):
        x = self.slider.val
        col = self.cmap(x)
        self.fax.set_facecolor(col)
        for line in self.lines:
            line.set_color(col)
        self.slider.poly.set_color(col)
        self.slider.vline.set_xdata(x)
        self.lax.set_xlabel(self.fmtcol(col))
        self.fig.canvas.draw_idle()

if __name__ == "__main__":
    if len(sys.argv) > 1:     
        CMViewer(sys.argv[1])
    else:
        CMViewer("RdYlBu")

If this is saved as CMViewer.py it could be run as script like

> python CMViewer.py 

or with a colormap specified,

> python CMViewer.py viridis