It's true that older mercury vapor lamps with fluorescent coating are green in color?

And why??

Mercury: Yes. Fluorescent: No.
Older mercury lamps had a clear finish, so the light came directly from the greenish mercury discharge. In later mercury lamps, a red phosphor is applied to correct the greenish color of the mercury discharge to white.
The first fluorescent lamp phosphor was Zinc Beryllium Silicate, which produced white light, but it was very inefficient and toxic. Later lamps used Calcium Halophosphate, which is the material of old T12 and T8 lamps from the 1990's and earlier. Newer lamps uses three rare-earth phosphor coatings in the red, green and blue colors, and achieves high color rendering white light.

The Mercury discharge produces clear Green/Blue-ish color. This color may be balanced out to different extents by the phosphor

With the newest phosphor option (over the last 40 years or so) - Yttrium Vanadate - The light color and brightness still varies with the quality, quantity, wear level of the phosphor and on the blackening of the arctube. Aging lamps tend to dim down due to arctube blackening, and sometimes turn green to different extents also from degradation of the phosphor. The latest lamps may be dimmer and greener from the start due to varying phosphor quality

One of the older phosphor generation was designed to emit mainly green intentionally to boost the efficacy (the green gives you most lumens for the same radiated power). But later the introduction of generally more efficient lamps (mainly HPS) and the mecoming cheaper to make, reduced the MV market to the higher quality white light applications, leaving the high efficacy low color quality market for both sodium variants.
So the following development went back along preffering the white balance. Plus the new phosphors (Yttrium Vanadate based,...) were more efficient so those products reached the same efficacy as the previous greenish high efficacy lines, so those greenish ones were discontinued altogether.
I'm convinced the new phosphor generation would be possible to mix so to reach even higher efficacy, but it would still be far lower than the sodium based lamps, so the MV development never went that way anymore, all new development went along the in the "get the best color, while still stay at the 50 lm/W mark".