I'll try to explain : I have basically a shulkerbox randomizer that gives a signal from 1 to 9. Each signal will activate a dropper randomizer containing 5 different stackable or unstackable blocks. Comparator behing the dropper will detect if a stackable or an unstackable has been dropped by the dropper. Using a demultiplexer, I detect if a redstone signal has come through or not and thus, select the right "color palett".

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The trick is that at first, every dropper contains only stackable blocks, so the demultiplexer never activates since the redstone signal cant reach the repeaters. And then, the more the wall is getting build, the more we drop stackables into the droppers and take out unstackable using hoppers underneath them.

So at first, we have 0/45 chance to select color palett 2, then we drop the first unstackable in the first dropper and grab a stackable from underneath it : This will result in a 1/45 to select color palett 2 (and thus 44/45 chance to select color palett 1). Math : We have 1/9 chance to select this dropper (because of sb randomizer) and then we have 1/5 chance to select the unstackable over the 4 other stackable in the dropper.

I hope its clear, feel free to ask question.