I am not Brooks -- but use the same techniques for effects. The key concepts are opacity and reflectance. If the pink is translucent and light gets through, the brass tube provides reflection through the pink. The black or other opaque paint prevents any light from reflecting off the tube and being seen.
Why paint the layers in the tube?? Because it blocks light from reflecting from the bubbles and streaks in the glue and best controls the light reflecting back and seen by the eye. Some color epoxy with paint to accomplish a similar outcome.
Layering paint and leaving streaks of different colors can create some dramatic color variation in translucent acrylic blanks and is worth experimentation - and the surprise of discoveries. Nails polishes are good for "streaking colors".
Bright crimson paint on the inside of a pink acrylic can make it "hot pink" shaded towards red streaks. Lots of choices