如何在pandas柱状图中按字母顺序排列x标签和更改颜色？

import pandas as pd
import matplotlib.pyplot as plt

data = {'Letter': ['C', 'A', 'D', 'B'],
        'Value': [3, 5, 2, 4]}
df = pd.DataFrame(data)

df = df.sort_values('Letter')

fig, ax = plt.subplots()
ax.bar(df['Letter'], df['Value'], color=['red', 'green', 'blue', 'yellow'])
plt.show()