I have been asking myself a similar question and spent some time looking for the answer to find in the end that this can quite easily be done by imshow:

from matplotlib import pyplot

pyplot.imshow([[0.,1.], [0.,1.]], 
  cmap = pyplot.cm.Greens, 
  interpolation = 'bicubic'
)

It is possible to specify a colormap, what interpolation to use and much more. One additional thing, I find very interesting, is the possibility to specify which part of the colormap to use. This is done by means of vmin and vmax:

pyplot.imshow([[64, 192], [64, 192]], 
  cmap = pyplot.cm.Greens, 
  interpolation = 'bicubic', 
  vmin = 0, vmax = 255
)

Inspired by this example

Additional Note:

I chose X = [[0.,1.], [0.,1.]] to make the gradient change from left to right. By setting the array to something like X = [[0.,0.], [1.,1.]], you get a gradient from top to bottom. In general, it is possible to specify the colour for each corner where in X = [[i00, i01],[i10, i11]], i00, i01, i10 and i11 specify colours for the upper-left, upper-right, lower-left and lower-right corners respectively. Increasing the size of X obviously allows to set colours for more specific points.