Game booted with garbled graphics and only blue colour was present.
I quickly found RAM U16 (type 6264 narrow) was extremely hot. I replaced it and graphics were restored but still blue only.
Starting from the edge connector I probed the board backward: no activity on the green and red pins of the edge connector, then no activity on the resistors used as DAC, and finally no activity on the outputs of one of the two LS273 used for colours. Input pins had activity as it should, clock seemed valid so as master reset. I pulled it and installed a new one, colours were back: