In your example, p is a list of Line2D object. In that example you have only one line object, p[0]. The following is an example plotting three lines. As more line is added, it is appended to the p. So if you want the color of the last plot, it will be p[-1].get_color().
import numpy as np
import matplotlib.pyplot as plt
x = np.arange(10)
y = np.arange(10)
p = plt.plot(x,y, x,y*2, x,y*3) # make three line plots
type(p) # list
type(p[0]) # <class 'matplotlib.lines.Line2D'>
p[0].get_color() # 'b'
p[1].get_color() # 'g'
p[2].get_color() # 'r'
