In 1958, buyers of a brand new 1958 Buick had the choice between 25 different base colors. They could be combined to a two-tone paint combinations. The three-tone paint was no longer available. Maybe this was due to the massive use of chrome!
Buick introduced a new kind of paint for the new model line called “Lucite” paint. A promotional flyer about the six Lucite colors said: Lucite is a completely new kind of car paint, with a fundamentally different base – related to glass-clear acyrlic resin – permitting dramatic color effects. It dries to a high, hard, finish that practically never needs waxing or polishing.
The body tag from your 1958 Buick tells you if your 58 Buick was painted in one (or two) of these new Lucite paints. The usual colors were identified by letters. The Lucite paints on the other hand had numbers. All two-tone combinations can be identified by the combination of letters and numbers. The first letter/number indicates the lower color and the second letter/number is the upper color.