Довольно просто, используя ListedColormap :
import matplotlib.cm as cm # color map
import matplotlib.gridspec as gridspec
import matplotlib as mpl
import numpy as np
#dummy data
hm = np.random.randint(0, 100, size=(5, 5))
#define colors
cmap = mpl.colors.ListedColormap(['white', '#FFFFBB', '#C3FDB8', '#B5EAAA', '#64E986', '#54C571',
'#4AA02C', '#347C17', '#347235', '#25383C', '#254117'])
#create plot frames
gs = gridspec.GridSpec(2,2, height_ratios=[6,2], width_ratios=[20,1],)
ax = plt.subplot(gs[0, 0])
#heatmap and colorbar
im = ax.pcolor(hm, cmap=cmap)
cbar = plt.colorbar(im, cax=plt.subplot(gs[1]))
plt.show()