nearest_color.py

from PIL import Image

'''
 	Black	(0,0,0,1)	(0,0,0)	#000000
 	White	(0,0,0,0)	(255,255,255)	#FFFFFF
 	Red	(0,1,1,0)	(255,0,0)	#FF0000
 	Green	(1,0,1,0)	(0,255,0)	#00FF00
 	Blue	(1,1,0,0)	(0,0,255)	#0000FF
 	Yellow	(0,0,1,0)	(255,255,0)	#FFFF00
 	Cyan	(1,0,0,0)	(0,255,255)	#00FFFF
 	Magenta	(0,1,0,0)	(255,0,255)	#FF00FF
'''

palette = [
    (0, 0, 0),
    (0, 0, 255),
    (0, 255, 0),
    (0, 255, 255),
    (255, 0, 0),
    (255, 0, 255),
    (255, 255, 0),
    (255, 255, 255),
]

def euclidDistance(p0, p1):
    deltaR = p1[0] - p0[0]
    deltaG = p1[1] - p0[1]
    deltaB = p1[2] - p0[2]
    return (deltaR ** 2 + deltaG ** 2 + deltaB ** 2) ** 0.5

def nearest(pixel, palette):
    nearest_distance = (255 ** 2 + 255 ** 2 + 255 ** 2) ** 0.5
    for i in range(len(palette)):
        color = palette[i]
        distance = euclidDistance(color, pixel)
        if distance < nearest_distance:
            nearest_distance = distance
            nearest_distance_index = i
    return palette[nearest_distance_index]

def traverse(image):
    width = image.size[0]
    height = image.size[1]
    for x in range(width):
        for y in range(height):
            pixel = image.getpixel((x, y))
            image.putpixel((x, y), nearest(pixel, palette))

image = Image.open("./lena.png")
target = image.copy()
traverse(target)
target.show()
target.save("./target.png")

Original Image

Processed Image