Is this what you mean?

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap

x = [1, 2, 3, 4, 5]
y1 = [1, 1, 2, 3, 5]
y2 = [0, 4, 2, 6, 8]
y3 = [1, 3, 5, 7, 9]

y = np.vstack([y1, y2, y3])

labels = ["Fibonacci", "Evens", "Odds"]

fig, ax = plt.subplots()

# Create a custom colormap
cmap = LinearSegmentedColormap.from_list("custom_cmap", ["green", "transparent"])

# Plot the first two areas normally
ax.stackplot(x, y1, y2, labels=labels[:2])

# Plot the "Odds" area with a gradient
for i in range(len(x) - 1):
    ax.fill_between(x[i:i+2], y1[i:i+2] + y2[i:i+2], y1[i:i+2] + y2[i:i+2] + y3[i:i+2], 
                    color=cmap(i / (len(x) - 1)))

ax.legend(loc='upper left')
plt.show()