I have also set the field fill to white via conditional formating, and this has worked even when the field was set to transparent (no fill) as its normal layout value. The conditional formatting then hides anything behind it, though I realize that if you want something else to show through it, and that would be another problem.
In that case, you might try setting the white text to also conditional format to 1pt size and see if that's useable for your needs.
This techniquie has worked in previous versions. In FM 12 I can see the data behind the text when the conditon of 500Px is met. The problem that is new in 12 is I have a large box appearing. I resized the text box (It was original just the size FM made it when I entered the text), and now it is working, kinda. It shows as grey when i click on it (it is a button), but now the gray is the same size as the button so it sorta works.
Not a direct answer to your question, but Bruce Robertson suggested awhile back, that this technique might not be the best. This was well before 12 arrived. You can search the archives, but the gist was that upon closer examination, it was linked to a definite hit in performance. I guess what I am suggesting is to consider the idea of coming up with something in its place such as what Stephen proposed.
Okay, since I don't have anything specific to suggest in its place, you can just call me "Debby Downer..." (from Saturday Night Live)
Its not always possible, but I try to set the text color to match the background in the conditional format. This works fairly well for me. With 12 this can be a bit more complex since you now have 4 states and the field can look different in each state.
I have also used this trick a lot. e.g. on lists with alternate background fill.
And also on textbuttons. Now in 12 when a button is gone - it is not - instead it has become a lot bigger.
Why isn't it possible to make text transparent via cond. form. ?
I guess the only solutions are cond formatting and have the color blend in with the background.
or making extra ui-fields to make the text disappear.
Or are there other better solutions out there ?