Finally got to look at this. On an arcade machine (such as pacman), the colour lines go through a resistor network to set the proper intensity of each colour. MAME has generic code to convert resistor values into the final colours.
However, according to the 256tc diagram (I couldn't find one for a colour 128k), the RGBI lines go directly to the "colour port", and it's up to the monitor to do the final mixing. Needless to say, this varies per monitor, and I don't have schematics for them.
I don't know what happened to Chickenman's testing, so I can't really do anything right now. But, should more info turn up, then this can be progressed.
EDIT: I am no longer involved in the MAME project, so this topic may as well be closed, because no fixes will be done.
However, according to the 256tc diagram (I couldn't find one for a colour 128k), the RGBI lines go directly to the "colour port", and it's up to the monitor to do the final mixing. Needless to say, this varies per monitor, and I don't have schematics for them.
I don't know what happened to Chickenman's testing, so I can't really do anything right now. But, should more info turn up, then this can be progressed.
EDIT: I am no longer involved in the MAME project, so this topic may as well be closed, because no fixes will be done.