You can change the colors in your stacked area chart with the colors parameter of the stackplot() function. Here I propose 2 solutions: the first one is to use a common color palette, the second is to pick up your colors one by one.
# libraries
import numpy as np
import matplotlib.pyplot as plt
import seaborn as sns
# Your x and y axis
x = range(1,6)
y = [ [10,4,6,5,3], [12,2,7,10,1], [8,18,5,7,6] ]
# use a known color palette
pal = sns.color_palette("Set1")
plt.stackplot(x,y, labels=['A','B','C'], colors=pal, alpha=0.4 )
plt.legend(loc='upper right')
plt.show()
# create your palette
pal = ["#9b59b6", "#e74c3c", "#34495e", "#2ecc71"]
plt.stackplot(x,y, labels=['A','B','C'], colors=pal, alpha=0.4 )
plt.legend(loc='upper right')
plt.show()
