What you need are green bulbs in there behind red lenses. Then they'll flash orange. Sounds odd, I know, but trust me, it works.
Green? Oh they'll be easy to find. Me thinks a long discussion with MOT tester about the good old days and a pair of Orange ones that "I'll fit later" will do the trick.
Clear green model paint from Humbrol should do the trick. If not wire the indicators to the reverse lights and use the original indicators as extra brake lights like this:
