There were some Hondas that used gold paint for the letters, just to make it more difficult...
White and silver being more common. The 400F that I've seen used white.
There were several bikes that had a two tone seat cover and the Honda was in black on a white vinyl. (C70 Passport and others)
Maybe edit the thread title to make it more model specific, adding 400F on the end or "What color/colour HONDA on back of 400F seat?".
MoMo has restored several 400F along the way. PM to him might net an answer...