Hmm, apparently the Apple PPC upgrade cards for the Quadras used a yellow PowerPC 601. linky
I honestly never knew that yellow chips were used in shipping products..I've only ever seen them in PR photos before, so I just assumed that all production model 601s were blue.
Good question. I'd guess that it likely functioned as the bridge between the 60x bus and the '040 bus, with the ARM core performing operations such as byte reordering or translation, and maybe handling the transition between the 68k and PPC environment via the control panel.