Dear Angler, thanks for the above question!
How to answer, is for sure a challenge for me.
I've just delivered a white car yesterday (!) and I'm doing my best to remember how I did to see polish properly working on white... and I'm known for having good memory!
Sorry, I can't give a precise / definitive answer because I just can't recall exactly, I'm somewhat adapted to menzerna polishes that nowadays this comes 'automatic'...
Monday / Yesterday I've used SF4000 on white car and maybe for the polish itself being 'silver' in color, it helped seeing the change so this difficult was not faced, I believe that's why I can't recall this issue you're relating.
It's like the polish film is 'heavier' on the start (sure, plenty of product), and then it start turning transparent and all you see is the gloss behind a very thin film. 'Turning transparent' may not be the correct description, it becomes 'oily' and glossy, thinner...
To illustrate, try for yourself (or just imagine the following situation):
Get a polisher and a polish, spread it over a work area. Work it with no pressure, and fast arm speed. The 'density' of polish film may take very long time to start showing any significative change, because you're just 'spreading polish' over surface.
Now take the same setup, and exerting enough pressure, using slow arm speed, start working the section. You'll notice that in some passes (generally) 3 - 4 passes things will be slightly different, since product is demonstrating signs of use (oily, glossy, thinner... like turning transparent).
Another indicator I use to feedback my process is the easiness to remove residue at the end. If it's a breeze to remove, it may also indicates product was worked properly (not a rule, since some products may be temperature / humidity sensitive).
On same white, I've worked spots with FG400 (some RIDS) which is a white product (that may cause the issue you're relating on white), but I've used it only for enough time to remove the defects (some 3 passes with slow movement and much pressure), wiped it off and then followed with SF4000 to finish area (this for the whole cycle),
Also, I'm used to tilt my head many times closely to paint I'm working, on angle, if I have difficult seeing what's happening.
I too use plenty of light or directed light to area I'm working.
From what I can remember, many times working on white yesterday I wasn't able to see anything after some passes, just like I was 'dry polishing'. I've had this sensation 2 or 3 times, I just stopped what I was doing, tilted head on angle and after seeing the polish film there (I thought, 'nevermind') I returned and keep polishing.
From what I can recall, I had difficult removing residue on white paint when I used PBL cleansing polish. This product simply disappeared from my eyes, and I was just able to wipe it off in a efficient manner using a swirl finder light to help the task (this on a previous work on white, not yesterday).
So yes, depending on product used it may be difficult seeing on white, some more, some less.
Based on my (little) experience, SF4000 is slightly easier to work on white being it's color silver and that may contrast with white paint. PBL cleansing (which is white and goes like a cream) is tricky to observe.
Please, let me know if you got an answer,
I've done my best to help you with whatever I could on this one.
Sorry for some explanations since I'm Brazilian and I have difficult expressing my thoughts correctly in foreign language.
See for yourself, stop at every 2 words to remind how to say ......... in english?! Hahha something like that.
Again, feel free to re-question if any more help is needed.
Also looking forward to hear some other guy comments on that.
I've tried, thank you for opportunity.
Kind Regards.