Humbrol enamel paint and a fine brush!
I have had three badge replacements and each has been worse that the original.
All of the Plastic fell out of the last one so I bought the Darkest Green in the Humbrol range, slapped it in with a fine brush, wiped off the excess and then cured it in a warm oven after Sundays cooking (Gave a nice rounding to the edges).
So far its still all there, so far although the green lightened from the tin lid colour so I may need to change it, eventually???