Tuples are ordered data containers, which cannot be modified after they are assigned.

import turtle
import random

turtle.speed(9)

colors = ("Green", "Blue", "Orange", "Pink", "Red", "Purple", "Magenta", "Cyan", "Yellow", "Black")

for i in range(100):
    color_index = random.randint(0, len(colors)-1)
    color = colors[color_index]
    turtle.pencolor(color)
    
    turtle.pensize(random.randint(2, 7))
    
    radius = random.randint(20, 200)
    turtle.circle(random.choice([-1, 1]) * radius)


turtle.done()

The random.randint() is a built-in function for obtaining a random integer number between the two numbers given as arguments, both included.

When accessing an element in a tuple, one has to provide an index number inside the pair of brackets following the name of the tuple variable. The first element of the tuple resides in position [0]. If there are n elements in a tuple, the last element resides in position [n-1].

The random.choice() method returns a single element from a sequence of items, including tuples.

The code section

    color_index = random.randint(0, len(colors)-1)
    color = colors[color_index]
    turtle.pencolor(color)

above can simply be replaced by

    turtle.pencolor(random.choice(colors))