Without the finite-area limit, this puzzle is much too easy.  I said as much to Haxton, while showing him an 18i solution I built in ten minutes.  Slappy stick tech is too well-established to give much room for magic here.

But with the limit? That's a while different kettle of fish.

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Berlo works most efficiently, in I solves, if it sees 2 atoms per 3 loops (1i), 2 atoms per 1 loop (3i), or 1 atom per 1 loop (2i + catalyst, min 4i).  But NONE of those numbers works well with a 7-atom input.  Without suppression or a LCM trackloop, the slowest we can produce atoms is 7 per 6 loops--too fast for two of those options, and requiring very awkward conditional center-atom handling for the last.

So, ruling out tracks, I decided early on that suppression was the answer, trying for 1 atom per loop.  Fortunately, Pentapig showed an excellent method for this in Aether Reactor.  Converting the bond structure of the input into a spiral allows a single arm to conditionally handle the center atom with only 1 extra instruction.  

In addition, Penta showed how to get all 4 colors from a 2i Berlo, which usually only cycles through 3 colors: build a catalyst from the 4th color before the wheel starts spinning, and conditionally apply that to every 4th atom.  He accomplished that with a length 4 trackloop; here, I manage it with 4-long sticks. 

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I feel good about my work here, but do see some weaknesses.  First, I am using the least instruction-efficient Berlo timing of the ones I mentioned.  Second, it costs me 7i to spiralize the inputs, which seems excessive.  Finally, using sticks to count to 4 costs one extra instruction, on account of the consequent need to arrange for debonding.  All told, it seems perfectly reasonable that these same ideas could go as low as 26 or so.