Where do you see this? The article talks about nesting levels being bounded by O(log N) levels where N is the length of the document, and from what I can tell this is just for analysis and it supports arbitrary nesting levels… Oh, is your question about the actual colours used for the brackets, i.e. arbitrary levels of matching are supported, but the colours repeat every 6 levels (e.g. brackets at levels i and i+6 have the same colour)? If so this is a UX choice (I can't see any technical reason) and I imagine the considerations might be something like:
- There are only finitely many colours that can be usefully distinguished visually by the average person, so we need to pick some limit L,
- The limit L needs to be small enough that all L colours are easily distinguishable and memorable, but large enough that it would never be confusing whether a certain coloured bracket is at nesting level i or i ± L.
I guess the thinking may have been that 6 is large enough typically, e.g. it's likely to be clear from context whether a certain bracket (say blue) is at (say) level 2 or level 8.