Here's a Python program that takes an image file and two colors,
to_rgb. It creates a new image that's equal to the input except that every pixel colored
from_rgb is replaced with
from PIL import Image
from_rgb = (255, 255, 255)
to_rgb = (255, 0, 0)
with Image.open(sys.argv) as im:
im.putdata([to_rgb if x == from_rgb else x for x in im.getdata()])
Use GIMP's Select by Color tool. Turn off antialiasing, set the threshold to 0, and click on a pixel with the color you want to find.
The marching ants in the below example are the result of clicking on one of the orange pixels.