IC5, VPAL, Video

VIDEO_CLK is simply half the 15 MHz clock frequency. The PAL can work in two modes.

In 640-pixel wide mode, two bits from the shift registers (VD1, VD0) are fed to T0-1 to drive a 2-bit-address 4-bit-data look-up table, acting as a colour palette.

In 320-pixel-wide mode, VD1, VD0 are shifted into RED, GREEN, BLUE and INTENSITY. 

BC0-3 is the Background Colour.

An LS157 multiplexer (IC3) selects either the RGBI bits from the PAL (IC5) or the palette (IC4). ENPAL selects which.

I don't think this would be very hard to reverse engineer. The worst case is that wrong colours appear on the screen, which won't harm the 1616 hardware and would usefully indicate what is going wrong.