For a ROM modification I searched a lot of default adresses in the C128 ROM map, here they are.
C64 Kernel Colors:
$E535 $0E - Cursor color
$EC09 $0E - Frame color
$ECDA $06 - Background color
C128 Kernel VIC Colors:
$0E85 $0D - Cursor color
$22E7 $0B - Frame color
$22E8 $0D - Background color
C128 Kernel VDC Colors:
$0E9F $07 - Cursor color cyan
$2323 $F0 - Frame + Background color black
$388C $A2 - Boot routine call during reset
$388D $08 - Device for boot routine
If you replace in address $338C the value $A2 (STA mnemonic) with $60 (RTS mnemonic), the boot routine will be skipped. The BOOT command is still working.
Modyfing the value in $338D will change the boot routine to another device. I'll use drive 17 for bootable MO media.
The main reason for researching these values was the dull black/cyan VDC color. It always looks too old-aged. So I changed it to blue/white, as in the C65.
These are the values for the VDC screen, the VIC color's weren't modified.
$0E9F $0F - Cursor color white
$2323 $F2 - Frame + Background color blue
With some more modifications in the kernal and the Basic-lo ROM it's possible to create a look which comes more close to a C65, take a look in the attachments. But it's only a ROM text hack, there are no detection routines implemented.
The complete default values for the VIC are located from $E2C7 to $E2F7, for the VDC Tab 1 from E2F8 to $E331.
The following bytes from $E332 to $E33A should be FF 19 47 FF 04 27 07 20 FF, the values are named as VDC Tab 2 and VDC Tab 3 in my ROM listing. $FF is a seperating character.
Playing around with this could be interesting to modify the VDC's setup during power on, so it'll be possible to move the screen position or changing synronization values. It's easy to test the modified ROM images in VICE using the Resource Inspector, but after changes you have to load another ROM file (f.e. the default one), reset, then re-load the file you've modified and reset again.