Hello from Everett, Washington USA.
I do not use CA glue for securing the brass tube to the blank, I have used 5-minute 2-part epoxy only, as I have a better success rate, especially with colored tubes. I have made close to 500 pens in the last 5 years and use epoxy 99% of the time, using CA to secure the blank to the tube on laser-cut inlaid blanks.
If you need a black colored tube, I have successfully used Birchwood Casey's BRASS BLACK. It is a liquid that, when applied with a cotton swab, turns the brass tube black by chemical reaction. It is used by gunsmiths and other tradespeople. They also have a product, ALUMINUM BLACK, that has the same effect on aluminum, and I have used it with good results on other craft projects over the years.
For other colors, I use enamel model paint or acrylic paint. On painted tubes, I have colored both the outside of the tube and the inside of the drilled hole with epoxy only with good results.
I do not know if these products are readily available outside of the US, but I purchased these supplies from Amazon,