code: def test_op_ndoverlay_value(self):
      colors = ["blue", "red"]
      overlay = NdOverlay(
          {
              color: Spikes(np.arange(i + 2))
              for i, color in enumerate(colors)
          },
          "Color",
      ).opts("Spikes", color="Color")
      plot = mpl_renderer.get_plot(overlay)
      colors = [(0, 0, 1, 1), (1, 0, 0, 1)]
      for subplot, color in zip(plot.subplots.values(), colors):
          children = subplot.handles["artist"].get_children()
          for c in children:
              self.assertEqual(c.get_facecolor(), color)
  
imports: import numpy
  import pytest
  from holoviews.core.options import AbbreviatedException
  from holoviews.core.overlay import NdOverlay
  from holoviews.core.spaces import HoloMap
  from holoviews.element import Spikes
  from ..utils import ParamLogStream
  from .test_plot import TestMPLPlot, mpl_renderer
label_desc: Use image node_id 19 for calling the TestSpikesPlot obj's underlying member method code with example usage: obj.test_op_ndoverlay_value() without return types