Turtle Module for Python - How would I make the turtle do an action according to a stamped color underneath it?

The Langton's Ant sequence never ends, hence it is impossible to contain the entire image in a turtle graphics window.

If you have a fixed number of steps you intend to allow the ant to make, then you can use this class that simulates the Langton's Ant sequence, and returns the dimensions of the grid you will need:

import turtle

SCREEN_WIDTH = 600 # You can customize this
SCREEN_HEIGHT = 600 # You can customize this
steps = 1000 # You can customize this

class LangtonsAnt:
    def __init__(self, steps):
        self.grid = [["AW"]]
        self.facing = 0
        self.steps = steps
        
    def current_index(self):
        for i, row in enumerate(self.grid):
            for j, cell in enumerate(row):
                if "A" in cell:
                    return i, j
                
    def switch_color(self, i, j):
        if "W" in self.grid[i][j]:
            self.grid[i][j] = "B"
        else:
            self.grid[i][j] = "W"
            
    def turn_cw(self):
        self.facing += 1
        if self.facing > 3:
            self.facing = 0
            
    def turn_ccw(self):
        self.facing -= 1
        if self.facing < 0:
            self.facing = 3
            
    def forward(self):
        i, j = self.current_index()
        self.switch_color(i, j)
        if self.facing == 0:
            if len(self.grid[i]) > j + 1:
                self.grid[i][j + 1] = "A" + self.grid[i][j + 1]
            else:
                self.grid[i].append("AW")
                for idx, row in enumerate(self.grid):
                    if idx != i:
                        row.append("W")
        elif self.facing == 1:
            if len(self.grid) > i + 1:
                self.grid[i + 1][j] = "A" + self.grid[i + 1][j]
            else:
                self.grid.append(["AW" if idx == j else "W" for idx in range(len(self.grid[i]))])
        elif self.facing == 2:
            if j:
                self.grid[i][j - 1] = "A" + self.grid[i][j - 1]
            else:
                self.grid[i] = ["AW"] + self.grid[i]
                for idx, row in enumerate(self.grid):
                    if idx != i:
                        row.insert(0, "W")
        elif self.facing == 3:
            if i:
                self.grid[i - 1][j] = "A" + self.grid[i - 1][j]
            else:
                self.grid.append(["AW" if idx == j else "W" for idx in range(len(self.grid[i]))])

    def current_color(self):
        for row in self.grid:
            for cell in row:
                if "A" in cell:
                    return cell[1]

    def dimensions(self):
        for _ in range(self.steps):
            self.forward()
            if self.current_color() == "W":
                self.turn_cw()
            else:
                self.turn_ccw()
        return len(self.grid[0]), len(self.grid)

wn = turtle.setup(SCREEN_WIDTH, SCREEN_HEIGHT)
test = LangtonsAnt(steps)
turtle.speed("fastest")
turtle.shape("square")
s = min(SCREEN_WIDTH, SCREEN_HEIGHT) / max(test.dimensions())
turtle.shapesize(s / 20, s / 20)
turtle.penup()
for i, v in enumerate(test.grid):
    for j, w in enumerate(v):
        turtle.goto((-SCREEN_WIDTH + s)/ 2 + j * s, (SCREEN_HEIGHT - s) / 2 - i * s)
        if "B" in w:
            turtle.stamp()

Output: