Use luminance to choose text colour over colour cycles

This uses the same algorithm as the [Specifying colors
tutorial](https://matplotlib.org/stable/users/explain/colors/colors.html#comparison-between-x11-css4-and-xkcd-colors).

scripts/colors.py

@@ -33,12 +33,19 @@
 for name, colors in palettes.items():
  C = mpl.colors.to_rgba_array(colors).reshape((1, len(colors), 4))
  ax.imshow(C, extent=[xmin, xmax, ymin, ymax])
+
+ # Drop alpha by assuming we're on a white background PDF.
+ alpha = C[0, :, 3]
+ rgb = C[0, :, :3] * alpha[:, np.newaxis] + (1 - alpha[:, np.newaxis])
+ # Same calculation for luminance as
+ # https://matplotlib.org/stable/users/explain/colors/colors.html#comparison-between-x11-css4-and-xkcd-colors
+ luma = 0.299 * rgb[:, 0] + 0.587 * rgb[:, 1] + 0.114 * rgb[:, 2]
+
  dx = (xmax-xmin)/len(colors)
  for i in range(len(colors)):
- color = "white"
- if colors[i] in ['1.0', 'w']: color = "black"
+ text_color = "black" if luma[i] > 0.5 else "white"
  text = str(colors[i]).replace(' ', '')
- ax.text((i+0.5)*dx, (ymin+ymax)/2, text, color=color, zorder=10,
+ ax.text((i+0.5)*dx, (ymin+ymax)/2, text, color=text_color, zorder=10,
  family="Source Code Pro", size=9, ha="center", va="center")
 
  fig.savefig(ROOT_DIR / f"figures/colors-{name}.pdf")