Highlight a particular value in X axis

I want to highlight particular values on x axis in Matplotlib .

list1=[1,2,3,4,5,6,7,8,9] - xaxis values 
list2=[1,2,23,4,5,6,7,8,9] - y-axis values 
list3=[2,3,6,7]

plt.plot(list1,list2)
plt.show()

I want to highlight the xaxis labels as well its corresponding values in any color like red/green or whatever only when list3 value is present in list1

One option, independently of the axis labels, could be to plot markers for the target values:

import numpy as np

m = np.isin(list1, list3)

plt.plot(list1, list2)
plt.plot(np.array(list1)[m], np.array(list2)[m], ls='', marker='x')

Output:

If you also want the labels:

m = np.isin(list1, list3)

ax = plt.subplot()
ax.plot(list1, list2)
ax.plot(np.array(list1)[m], np.array(list2)[m], ls='', marker='x')

S = set(list3)
for tick in ax.get_xticklabels():
    if int(tick.get_text()) in S:
        tick.set_color('r')

Output:

You could try using matplotlib.pyplot.annotate or axvspan. These are 3 possible solution I could think of.

import matplotlib.pyplot as plt
list1=[1,2,3,4,5,6,7,8,9] #- xaxis values 
list2=[1,2,23,4,5,6,7,8,9] #- y-axis values 
list3=[2,3,6,7]

plt.plot(list1,list2)

for value in list3:
    index = list1.index(value)
    x = list1[index]
    y = list2[index]
    plt.annotate("X",(x,y),xytext=(x-.1,y-.25), color="red")
    plt.annotate("X",(x,y),xytext=(x-.1,-0.5), color="red")
    plt.axvspan(list1[index]-.1, list1[index]+.1, alpha=0.3, color="green")
plt.show()

In this case in the xytext I substracted 0.1 and 0.25 (or -0.5) just for better overlapping of the "X" since this method is used for text annotation. In the case of axvspan I used +/-0.1 of the coloring range.

Hope it helps!