I want to remove all beige of the image

import cv2 
import numpy as np

# Read image
img = cv2.imread('brain.png')
hh, ww = img.shape[:2]

# threshold on white
# Define lower and uppper limits
lower = np.array([90, 120, 200])
upper = np.array([170, 200, 255])

# Create mask to only select black
thresh = cv2.inRange(img, lower, upper)

# invert threshold image
mask = 255 - thresh

# apply mask to image
result = cv2.bitwise_and(img, img, mask=mask)

# save results
cv2.imwrite('1_thresh.jpg', thresh)
cv2.imwrite('1_result.jpg', result)

cv2.imshow('thresh', thresh)
cv2.imshow('mask', mask)
cv2.imshow('result', result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Threshold Image:

Result: