When an HPS lamp first turns on, it is blue from the mercury inside used as a starter. It then slowly shifts towards yellow, and somewhere in between it just looks white for a few seconds.

Obviously if you have a metal halide lamp with just sodium iodide, the light emitted will be yellow when it is all warmed up. But, what if you only gave the lamp a controlled small dose of sodium iodide, so when it all evaporates it just makes white light (mixing with the existing mercury discharge)?

This might be less efficient, but would this theoretically work?

That is all wrong.

Blue-white you see from a cold sodium lamp is mostly from xenon buffer gas.

HID lamp just won't start properly without a buffer gas, Ar, Xe or whatever. Lamps having some metallic mercury may have a chance, but at room temperature mercury vapor pressure is still too low. With an arc tube filled with just NaI, good luck striking an arc in a hard vacuum! (evaporation temp of NaI at 1 Bar is 1300C!)

And there are lamps containing Hg + NaI + Ar buffer. Orange metal halides. In practice they start up with mercury-lamp like color with a tiny bit of sodium (evaporated from electrodes I believe)  and then turn pinkish orange, actual color of sodium + mercury in that chemistry.

@Multisubject: Here is a video of my sodium iodide lamp warming up and hot restriking: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-257138
And sodium iodide have, unfortunately, lower vapor pressure than metallic sodium. But it is less corrosive to the quartz than metallic sodium.

@RRK
My idea isn't starting the lamp in hard vacuum, I meant that the lamp would have a mercury fill with a limited dose of NaI.

@dor123
That is an interesting video, you can see at the time stamp 1:15 the white color I am looking for.

With a limited dose of NaI, you won't get an orange color at all, even on 400W.
Regarding to my 70W NaI lamp: It is an interesting "Commercial prototype compact orange MH lamp". As the mercury pressure in low wattage MH lamps is much higher than in >250W MH lamps, combined with the limited vapor pressure of NaI compared to metallic sodium, the mercury lines bleaches the light color of the NaI, making it similar to a warm white halophosphor fluorescent lamp, or even similar to GE Reveal fluorescent lamps.
See here for the spectrum: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-244905
See here for the overall light color: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-257177

@RRK
My idea isn't starting the lamp in hard vacuum, I meant that the lamp would have a mercury fill with a limited dose of NaI.

So, these are available commercially as orange MH made by BLV. Partial pressure of Na is lower than in a true HPS, so orange Na peak @589nm is less inverted and more narrowband. 70-400W euro HPS compatible. Actual color is not so pure orange, somewhat pinkish like old 3K CCT halo fluorescent, but still quite relaxing.

I collected some, recenly bought a box of four 70W's 'just in case' )


Have not checked carefully, but these lamps are probably saturated, having some remaining pool of liquid NaI.