You cannot currently auto assign a unique background color per high cardinality value in a Quick table, but you can get close using two approaches, built‑in row banding, or a calculated band field used in conditional formatting.
Open the table visual → Format visual → Cells. Under Background, turn on Alternate background colors between rows, then set Odd rows and Even rows to two subtle colors.
Please refer to the below Quick documentation and community post this might be helpful for you.