The best method you will find for colorizing something is to do it by hand and as you found out that is very tedious and means a lot of trial and error and saves. As NOYB pointed out that pic you posted has a lot of compression artifacts which will make you colourizing a lot harder to do. That aside heres what I managed to pull off messing about with photoshop for a little while.CraigMB said:
edit ---- had no clue what clour to make the eyes