Python Turtle colour shades

Question

I have some simple code to draw some squares. What I want to do is to fill each square with a progressively darker shade of grey (so first square is filled in white, the next one slightly darker grey, and so forth). How can I do this, within the code below? Any help would be much appreciated. Thank you. Brandon

import turtle

for count in range(15):

turtle.color("black", "grey45")

turtle.begin_fill()

turtle.left(20)

turtle.forward(100)

turtle.left(90)

turtle.forward(100)

turtle.left(90)

turtle.forward(100)

turtle.left(90)

turtle.forward(100)

turtle.left(90)

turtle.end_fill()

Answer 1

import turtle

turtle.color("black", "white")

turtle.colormode(1.0)

SQUARES = 18

SIDE = 100

shade = 1.0

for count in range(SQUARES):

    turtle.fillcolor(shade, shade, shade)

    turtle.begin_fill()

    turtle.left(360 // SQUARES)

    for side in range(4):

        turtle.forward(SIDE)

        turtle.left(90)

    turtle.end_fill()

    shade -= turtle.colormode() / float(SQUARES)

turtle.done()