Well that's actually very interesting! I wonder if it was the multiple flushes/booster actuation, or the fluid itself.
No! The accumulator dumps into reservoir. So even in we do not flush accumulator 5 times, it does so over time anyway.
I do it 5 times, to get as much, old fluid out of system as I can. That way, if new fluid darkens quickly. I know it's not from old remains in accumulator. This is important, to get a glue as to health of seals of master and condition of screens in ABS unit.
Flushing regularly is the most important. Procedure and what we use to do so, is second.
I too, once though all DOT III the same. A Toyota shop foreman said to me he thought the same, until a brake master recall of 05-07 models and related TSB. At that time I elected to use a top name off the shelve DOT 3 (Castrol). It was to flush-out the factory fill from my 2001, at ~8 year old (~2009). The factory fluid came out looking as clear as new, for which I was amazed at how good it look. 3 yrs latar, I flushed out the off the shelve brake fluid. It came out green from the calipers. 98-02 have a copper banjo bolt and washers. Copper turns green in presence of moisture. Older Toyota fluid clear after more than 8 yrs, non Toyota green in 3 yrs Hummm!
Than in ~2021. A W/S parts department friend, at Toyota. Told me, they're seeing unusually high number of master being sold, nationally. About that time, all our master became available and prices drop by ~$1k. Humm!
Use whatever you like. I flush more fluid through than most and I use only Toyota fluid. Cost maybe $20 or $30 more per flush.
Use whatever you like. I use only Toyota!