Make Seaborn color palette darker

I already have a custom Seaborn color palette that I use, e.g.

pkmn_type_colors_palette = {
    'Grass': '#78C850',
    'Fire': '#F08030',
    'Water': '#6890F0',
    'Bug': '#A8B820',
    'Normal': '#A8A878',
    'Poison': '#A040A0',
    'Electric': '#F8D030',
    'Ground': '#E0C068',
    'Fairy': '#EE99AC',
    'Fighting': '#C03028',
    'Psychic': '#F85888',
    'Rock': '#B8A038',
    'Ghost': '#705898',
    'Ice': '#98D8D8',
    'Dragon': '#7038F8',
    'Dark': sns.xkcd_rgb['dark indigo'],
    'Flying': sns.xkcd_rgb['sky'],
    'Steel': sns.xkcd_rgb['steel'],
}


sns.violinplot(x='Type1', hue='Type1', y='Defense', data=poke_df, palette=pkmn_type_colors_palette)

which gives a plot like

is there a way to make the whole color palette darker or lighter? but keep the mappings

You could convert the color to the hsv color model and change the value (brightness) closer to 0:

import matplotlib.pyplot as plt
from matplotlib.colors import rgb_to_hsv, hsv_to_rgb, to_rgb
import seaborn as sns
import pandas as pd
import numpy as np

def darken(color):
    hue, saturation, value = rgb_to_hsv(to_rgb(color))
    return hsv_to_rgb((hue, saturation, value * 0.6))

pkmn_type_colors_palette = {
    'Grass': '#78C850',
    'Fire': '#F08030',
    'Water': '#6890F0',
    'Bug': '#A8B820',
    'Normal': '#A8A878',
    'Poison': '#A040A0',
    'Electric': '#F8D030',
    'Ground': '#E0C068',
    'Fairy': '#EE99AC',
    'Fighting': '#C03028',
    'Psychic': '#F85888',
    'Rock': '#B8A038',
    'Ghost': '#705898',
    'Ice': '#98D8D8',
    'Dragon': '#7038F8',
    'Dark': sns.xkcd_rgb['dark indigo'],
    'Flying': sns.xkcd_rgb['sky'],
    'Steel': sns.xkcd_rgb['steel']
}

poke_df = pd.DataFrame({'Type1': np.repeat(list(pkmn_type_colors_palette), 20)})
poke_df['Defense'] = np.random.randn(len(poke_df)).cumsum()
plt.figure(figsize=(11, 4))
dark_palette = {key: darken(pkmn_type_colors_palette[key]) for key in pkmn_type_colors_palette}
sns.violinplot(x='Type1', hue='Type1', y='Defense', data=poke_df, palette=dark_palette)
plt.tight_layout()
plt.show()