Got a Black Tiger for a repair which had a colour problem on some graphics
It lacked some shades of colour
I decided to look around the palette rams @6C and 6D until I found a fixed address line
Tracing back I found an 74ls157 @5D with an output stuck low.
After checking the inputs I found one of them stuck low and according to its truth table, the output is always low.
Tracing back I found a 74ls273 witth Q3 output low and D3 input oscillating.
Piggybacking a good one restored the correct colours.
After changing it , the game was completely fixed