Yes I know caution yellow was a thing, But that was not an actual phosphor it was just a coloured glass or whatever, I am thinking more along the lines of DX type phosphors that increase light output n' such, Why did they never make other formulations of it to make greens blues and reds and other colours like that? it seems like they'd function basically like fluro lamps on steroids.

I once asked a similar question about using tri-phosphors in MV lamps to make them more efficient, but from what I understand, tri-phosphor won`t stand the heat from the discharge like Vanadate does, causing the lamp to blacken quickly
But I reckon mercury lamps would still be in use today if they could have found a tri-phosphor that was compatible, giving the classic MV lamps an efficiency of a modern fluorescent tube!

I guess coloured phosphors would suffer the same fate sealed in bulb with a 1300 degrees burner right next to it !, as its only the low pressure discharge their exposed to in a tube.

If you would fill an empty MV bulb with triphosphor, then fill it with argon/mercury and put an electrode at the top and bottom you would have a fluorescent bulb right?

Might work ok with 80W arctube in 1kW size outer bulb. Also, QL (Philips) did pack Triphosphors with higher power in not too big bulb somehow

I also understand that an LED filament lamp, (using diodes in UV blue), mounted inside a phosphor coated outer bulb, similar to MV had been tested by Sylvania, but nothing more ever became of it?

There is significant difference between fluorescent vs MV in the task the phosphor is supposed to do:
In fluorescents it is supposed to use the primary vacuum UV (around 250nm), generated by the low pressure arc, and then generate all components of the visible light from that energy. Only the green and blue are to be slightly less intensive from the phosphor, because they are generated by the discharge.
The high pressure MV arc absorbs practically all of shorter wave UV, what remains is mainly visible green, blue and UVA. Therefore the phosphor task is just eat up the UVA and generate mainly the red out of, plus with a modern high efficiency phosphors a bit of the other visible to balance the color.
The problem with MV and efficacy is, the efficiency of the discharge itself in generating any radiation (except IR) is way too low (just a bit above 10%), what is the fundamental limitting factor in MV efficacy (around 50lm/W, with CRI not exceeding 70)
Low pressure discharge in the fluorescents converts about 70..90% of the electrical input power into the UV radiation utilized by the phosphor mix. So even when the conversion losses of the phosphor are high (due to the high energy of the exciting UV and low energy of the radiated photons), the overall efficiency is not that difficult to maintain in the 25..30% range, yielding the 80..100lm/W efficacies with CRI in the 80's.