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 = bokeh_renderer.get_plot(overlay)
      for subplot, color in zip(plot.subplots.values(), colors):
          self.assertEqual(subplot.handles["glyph"].line_color, color)
  
imports: import datetime
  import numpy
  from bokeh.models import CategoricalColorMapper, LinearColorMapper
  from holoviews.core import NdOverlay
  from holoviews.element import Spikes
  from holoviews.plotting.bokeh.util import property_to_dict
  from ..utils import ParamLogStream
  from .test_plot import TestBokehPlot, bokeh_renderer
label_desc: Use image node_id 24 for calling the TestSpikesPlot obj's underlying member method code with example usage: obj.test_op_ndoverlay_value() without return types