As I remember - Black is always ground. which is why there are several on the connector. Red is 5V and yellow is 12V, or both should be. The colors do not always line up but that is what it should the be on the color scheme. You can check it with a multimeter - checking the header with the chips set on resistance without the machine being on. Most Chips have a 5V on the first couple pins and ground on the opposite end/side.

CC- the upgrade is simple with Mac 512Ke/Plus ROMs so it it is a 512Ke, then the ROMs do not need to be upgraded. The problem is how the address & data pins go to the SCSI Chip. I'm sure if there is a memory map of the Mac 128K/512K/Plus, one can figure it out. From this, you can build a daughter board with three sockets that goes on to the ROM Sockets with two for the ROMS and the third for the SCSI Chip.