evilbob,
A few thoughts on the original Mini...
THE reference book is The Original Mini restoration guge byAnders Ditlev Clausager. If it ever happened to a Mini this book will have a reference to it.
One popular option was the British Flag (Union Jack) painted on the roof and British Racing Green was the second most popular colour choice after red.
The truly psychodelic colours came later in the decade but many early Mini's were repainted to match the mood of the times so feel free to get all the groovy shades of the day.
Also there were available a number of "flower power" adhesive stckers in the amazing colours of lime green and purple and others shaped like flowers which were very popular after market accessories.
Think Carnaby Street , Austin Powers and Jimi Hendrix.
If you decide to really get into it and buy retro flared pants please do not post about it here!
Good luck, Cheers , Pete.