Nigel, Mark sums it up really well here.
I use a piece of cotton wool on the end of a BBQ skewer, dabbed in a little acrylic artists paint and swab the inside of the drilled blank. Allow to dry for at least overnight then glue with two part epoxy (other flies are available) but I like epoxy as you can also mix a very small amount of paint in with it to colour the glue just in case you have missed any with the paint.
You have probably guessed that I have been there ! From my experience it hasn’t ever changed how the epoxy sticks or sets so I have either been lucky or it’s waiting to bite me!
As Mark eluded too, the colour of paint will make a big difference to the look and colour of the lighter blanks. It’s trial and error. One thing I have noticed is that with lighter colours of blues and greens then use a lighter colour paint. And with white make sure it’s white and black for black etc, etc..
Hope that helps, have fun and enjoy.