how to know metrix(x,y) of color in the image with OpenCV

[kiderburm]

i would like to know position of color
EX. i draw rectangle (10,10) to (256,256) green color
and i print(img[10,10]) >> [255,0,0]
but i need to know where position is [255,0,0]
if i have the image some point is green but i don't know position


import cv2
import numpy as np
img = cv2.imread('BG-Black.jpg')

cv2.rectangle(img,(10,10),(256,256),(0,0,255),-1)
cv2.circle(img,(10,10),2,(255,0,0),-1)
cv2.circle(img,(10,256),2,(255,0,0),-1)
cv2.circle(img,(256,10),2,(255,0,0),-1)
cv2.circle(img,(256,256),2,(255,0,0),-1)

print((img[20,20]))

print([0,0,255])

cv2.imshow("img",img)
cv2.waitKey()
cv2.destroyAllWindows()