I would go with 175, but currents are similar so if you already have 150 that should work similarly. Just measure and make sure current is reasonable once it's run up regardless of what you choose (aiming for around 1.5A for 150W and 1.55A for 175, doesn't have to be exact).

So I tried two 175 watt lamps, and it never made it out of the mercury color stage. It ran for over 10 minutes. Am I doing something wrong? It started up just fine.

That is quite odd considering the voltage of those two lamps in series should be lower than the typical lamps it's supposed to run. Did you get the current? The ballast could be intentionally underdriving lamps for the purposes of using less copper (didn't think this would happen for VHO lamp ballasts but it is very common for normal lamp ballasts). Maybe if you have some 150Ws you can try them but I think the ballast may be the problem.

So I tried two 175 watt lamps, and it never made it out of the mercury color stage. It ran for over 10 minutes. Am I doing something wrong? It started up just fine.

what ballast are you using, it actually a proper VHO ballast? have you verfied the ballast itself is good? as mentioned by Multisubject if you have a multimeter try measure the lamp current

I have a HO ballast. What is the difference between the two? I don’t think I have a proper VHO ballast. Where can you get one?

(Assuming T12) HO is only 0.8-1A, while VHO is 1.45-1.52A. Definitely not suitable for 150W or 175W lamps. What exact ballast are you using? You might be able to use 70W or 100W MH depending on it's specs. You'll need an ignitor, but that is very simple with rapid-start ballasts.

It was a commercial GE F96 Ballast. It says 2N at the top. It is rapid start, and can run 3 lamps.

Could we see a picture of it? F96 could mean F96T8, F96T12, or the HO or VHO versions of either. A 3 lamp ballast doesn't inspire confidence but we shall see. With a picture of the ballast label it should be easy to determine its HID possibilities

Here it is. I don’t even think it is HO.

Here it is. I don’t even think it is HO.

yeah thats not going to work at all, as you say its not even a HO ballast, and to make matters worse, its an electronic HF ballast, so theres risk of arc tube resonance and explosion

really I would just advise getting a second metal halide ballast and running that extra wire and do things properly

Sounds good. I was told they didn’t want the lamps to be halide, they said it was “too dim” 

If you got a HO ballast then all you really can do is run two 100 watt mercury vapor lamp in series

Well this chart will come in handy, as I have a good use case for this now. We were also trying to light the back of our shed area, and I primarily use HPS so that we have a nice amber glow. Well, I have two barn lights ( decorative ) and I still can’t afford to have two ballasts for that area. So I am going to take my 150 watt MH ballast and throw in two 70 watt HPS lamps so it can light up the area nicely while only using one ballast. I know it sounds a bit janky, but it’s the best I can do at the moment. I will be using a ISB Sola Basic M102 150 watt MH ballast. How far could it remotely start a lamp? In my experience, I had a 15 foot wire with a 100 watt version and it started nicely.

As far as I know, if you use a standard ignitor, the maximum starting distance from ignitor to lamp would be around 2-10 feet. Interestingly, if you use an Advance LI533-LR Xtenza ignitor, it can be used to start lamps at a maximum distance of up to 50 feet!!!

Here is one for sale on eBay that I have found:

https://ebay.io/m/cxKVpd

Well now that’s an ignitor. Really cool, hope I don’t need it though. I appreciate it! If it didn’t work, at least I have a fix. What would that realistically be needed for commercially. Is what I’m doing more common back in the day?