我不太确定这是否属于不知道颜色图的内部结构"，但也许这样的事情可以添加线性 alpha到现有的颜色图?

I'm not quite sure if this qualifies within "not knowing the inner structure of the colormap", but perhaps something like this would work to add a linear alpha to an existing colormap?

import numpy as np
import matplotlib.pylab as pl
from matplotlib.colors import ListedColormap

# Random data
data1 = np.random.random((4,4))

# Choose colormap
cmap = pl.cm.RdBu

# Get the colormap colors
my_cmap = cmap(np.arange(cmap.N))

# Set alpha
my_cmap[:,-1] = np.linspace(0, 1, cmap.N)

# Create new colormap
my_cmap = ListedColormap(my_cmap)

pl.figure()
pl.subplot(121)
pl.pcolormesh(data1, cmap=pl.cm.RdBu)
pl.colorbar()

pl.subplot(122)
pl.pcolormesh(data1, cmap=my_cmap)
pl.colorbar()