Thanks to Laura, I now have re-read the Tidball book.
What I remembered was wrong (or came from another book altogether).
Tidball does indeed advocate three colours, but in a way different than I thought:
(easier to scan than to type it...)
A bit later she also writes that she uses "a heavy dark and two fine light colors".
So let’s go back some. Here is the profile, again:
Here it is, the threading again converted according to Tidball’s system. I don’t know what she means with “follow the rythm”, but this is a guess.
Using Tidball’s treadling system (as above – assuming that "block A" is the leftmost), and the recommendations to use one dark heavy weft and two fine light wefts, it looks like this:
(Again, I rotated to make it bigger in the picture-wiever)
Note that, according to the treadling specifications, there is one extra light-coloured pick "for the transition", which means there are two same-coloured picks at the block changes.
(But why the two light colours?)
For truth’s sake, I should also quote another sentence: "Weaving as drawn in is not commonly attemped in crackle, though the Swedish books contain many crackle (called Jämtlandsvaev) drafts for elaborate symmetrical patterns, symmetrically woven." (Me, the dumb Swede, can’t se any obvious relation between symmetric draft, symmetrically woven, and "as drawn in"... And I still haven’t seen any Swedish jämtlandsdräll woven any other way than "as overshot".)
However. Several of the (north american) weaving softwares often want to use tromp-as-writ, if left to their “automatic” preferences. So the mystery still remains: why tromp-as-writ?