I got this game in a lot of faulty boards from ekorz. I'd like to thank him a lot, the boards were in great condition. According to him romboard was fine but motherboard had colour issue and a repair had been previously attempted. I could confirm both, game had a saturated blue colour, and both palette RAMs and a couple of nearby TTL chips had been put on sockets.
I started by probing the palette RAMs and found signal /WE of IC42 was floating. I traced it back to a LS244 (IC40) but piggybacking it made no difference. Then I found the two enable signals (1G/2G) of this LS244 were tied together and held high when not writting to the RAMs. This causes the output signals to go in high impedance state, however probing /WE signal of the second palette RAM revealed it was high so there clearly was a pull-up resistor involved. I measured 1kΩ resistance with Vcc whereas the floating output measure few MΩ. Looking at the board closely I discovered a 1kΩ resistor between the two palette RAMs. On was tied to Vcc as expected the the other, altough aligned with the /WE pin of the RAM IC42 was connected to nothing. I could see the trace was going straight to the rivet (only few millimetres away) of /WE pin so to me had to be connected there. I restored connection and colours were fixed:
I then tested inputs, sound, etc. but found nothing else wrong.
Game (very easily) fixed.