Saving Python PILs in Index Mode

I have a question about what to do when I read an index mode image in Python's image processing library PIL.
Read the png (index mode) image and change the pixel value as shown in the code below. Attempted to save in index mode again.
However, the output image was not colored and was black and white.
The color information is missing, but I don't know what's wrong.
Does anyone know?
I look forward to hearing from you.
Thank you for your cooperation.

 from PIL import Image
import numpy as np
import sys

origin=(0,1,4,4,5)
to=(2,4,3,4,2)

for ID in open(sys.argv[1]):
   ID = ID.strip()
   im = Image.open (ID + '.png')
   imnp=np.array(im)
   for(i,j)in zip(origin,to): 
       imnp[imnp==i] = j
   pil_img=Image.fromarray(np.uint8(imnp), mode="P")
   pil_img.save(ID+'_con.png', 'png')