from PIL import Image
from PIL import ImageFilter
import numpy as np

img = Image.open('poruka_u_boci.bmp')
rgb = img.convert('RGB')

orig_color = (0,0,255)
replacement_color = (255,255,255)

data = np.array(rgb)
data[(data == orig_color).all(axis = -1)] = replacement_color
img2 = Image.fromarray(data, mode='RGB')
img2.show()