Reuse automatic color

# Reuse the automatically selected color from a plot
line2d = plt.plot([0, 1], [0, 1], ':')
plt.scatter([0, 1], [0, 1], c=line2d[0].get_color())

# Reuse the automatically selected color from a scatter
pathcol = plt.scatter([0, 1], [0, 1])
plt.plot([0, 1], [0, 1], c=pathcol.get_fc()[0], ls=':')

# Use the j-th default color
c = plt.rcParams['axes.prop_cycle'].by_key()['color'][j]

# Specify a default color explicitly via Ck
for k, y in enumerate(...):
  plt.plot(x, y, '.', c=f'C{k}')
  plt.plot([], [], '-', c=f'C{k}', label=str(k))