I've had a quick look through my references, and so far, no evidence of 'brown' replacing yellow numerals, although this colour was seen by late 1940, if perhaps rarely. However, I think I spotted a reference, on one of the crash reports in the new book we've both just got (Luftwaffe Crash Archive, Vol 1) to a 'brown' numeral, but I haven't yet closely studied every entry, and therefore, at this stage, don't know which JG was involved.