I think too many colors can be challenging to digest, especially when they are a similar size as entry-category colorings.  Thats why I was suggesting the cell border colorings.
When you say there may be a long streak of high/low days…. do you mean there may be several says (presumably in a row) which all have the lowest amount and same for highest?
I was originally thinking to just color the first low mark and first high mark… however, if there are other cells with those same balances – they should probably all be colored whether they are in a contiguous block or not.

I may be able to get really fancy and if there is a block of high/lows as described above – to only outline the block, not each cell (remove the colored borders between the cells in the block).