There are some more problems: There seems to have been two shades of RLM 81, the "normal" Braunviolett and a darker chocolate brown. And if you consider the possibility of fading, dirt etc. it's impossible to tell which color looks darker.
It's pretty difficult even if you have color photos. Here's a good example:
http://cgi.ebay.de/ws/eBayISAPI.dll?...tem=6165909543.
Was the Do 335 painted RLM 81/82 or RLM 70/71 like the He 111 in the background?