Inherited Characteristics from the TWOK and TNG-era uniforms

The garment itself was basically a turtleneck made of cotton lycra, and the color varied considerably over the years, as we'll demonstrate later. 

The pattern itself is alleged to have been a recycled TWOK undershirt/division shirt pattern, sans quilting and fitting darts, and with a different back zipper closure. We're not sure if that's true or not, but the similarity of the garments is apparent:

A turtleneck is a fairly simple garment, so there's no need to over-analyze the TWOK and VOY undershirts while comparing and contrasting. 

However, we will point out that, while the two garments were made from different, and different color, fabrics, the differences were otherwise relatively minor; the TWOK undershirt had a quilted collar and quilted sleeve cuffs, whereas the VOY undershirt did not, and the TWOK closed up the center back with an overlapping closure whereas the VOY undershirt's center back closure didn't. 

In other words, they were both "turtlenecks" that zipped up the back via a hidden closure, with fairly snug collars and sleeve cuffs. To get from TWOK to VOY one need only change fabric, omit the quilting and fitting darts, and close up the center back with no overlap, so it's entirely possible the TWOK undershirt was a direct ancestor of the VOY undershirt. 

The VOY undershirt also inherited one major aspect from the TNG-era uniforms: its color. 

Gray had been introduced into the "standard Starfleet palette" the year before Deep Space Nine launched with Captain Picard's "casual uniform," in TNG's fifth season: 

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