After git bisecting the code I found that starting from commit b9ab3f5 there is a issue when converting GIFs from "P" format to either "RGB" or "RGBA".
In this case, pillow generates a black and white only image as it can be seen further down.
I'll try to understand what's happening and make a pull request latter.

This little snippet shows a way to reproduce it:

from PIL import Image
from urllib2 import urlopen
from io import BytesIO

imgbuffer = urlopen('http://www.eecs.berkeley.edu/~loarie/test.colors.gif').read()

img = Image.open(BytesIO(imgbuffer))
rgbimg = img.convert('RGB')
rgbimg.save(open('tmp.gif', 'w'), 'GIF')

Result:

Expected:

cc: commit author @d-schmidt