Is the following a VIC hardware emulation accuracy issue, or is its 16-color palette determined by an internal driver table? To reproduce:
$ mame64 vic20
... press Scroll Lock for FULL keyboard emulation ...
... hold PC TAB key down as CTRL and press 8 to change cursor to YELLOW ...
The VIC border should match the cursor color changed above, but the VIC screen should be a Light Yellow color such that the 2nd POKE statement remains visible (poorly, but visible).
The VIC 20 doesn't have much color to offer us old 8-bit gamers. Taking one of those crayons away from a kid is sad.