ben88:
Welcome aboard, Sir!
That yellow-brown color which you are seeing is a combination of tobacco tar and sulfur, the latter which is a component of Vulcanite, and which rises to the surface as it oxidizes. I scrub such stems with Bon Ami cleanser applied to a piece of paper towling moistened with warm water, and use an old toothbrush to work it it into the nooks and crannies, especially around the lips, and any tooth indentations.
Once the stems are thoroughly clean inside and out, I use micro-mesh pads moistened with warm water to progressively polish them to shiny blackness. Even the most badly oxidized Vulcanite stem can be restored in this manner.
Good luck!
p.s.:
"pipe cleaning. help needed..."
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