..OK, here's how.

Credits:
Full thanks to cato.boss in the Czech Republic for researching and experimenting with this problem. cato.boss is a great guy and very helpful.

I can only take credit for writing the instructions below. And here they are...


Brief Instructions:
1. Downgrade the nav to v24 OS, so we can force an upgrade later on.
2. Make a special v28 OS disc where the ONLY change is the mono navboot file is replaced with the colour navboot file
3. Load new v28 special disc

Full Instructions

1. DOWNGRADE TO V24
Make yourself a good, unaltered, v24 OS disc.

Force the Mk4 Mono nav to upgrade to v24...
Insert nav in car, turn key on, let nav boot up (you cannot see anything, just wait 2 min)

Insert v24 OS disc. Nav will load v24 software. Wait for disc to be ejected. Remove disc.

Nav is now at v24. Turn key off, go and make a new disc...

2. MAKE V28-SPECIAL DISC
Get a genuine, unaltered, v28 OS image.

Open with MagicISO or your favourite image editor

Replace this file:
V_2 RR 0101 BMWM01S app_sw navboot

with this one:
V_2 RR 0101 BMWC01S app_sw navboot

...so that both the BMWM01S (monochrome) and BMWC01S (colour) directories have the same (colour) navboot file (11,807,976 bytes)
What we have done is put the colour navboot file in the monochrome directory, the mono nav will boot using the colour navboot, and switch to colour mode.

Burn a new v28-special OS CD. Label it as 'v28 special'

Verify that the two navboot files are identical (in the BMWC01S and BMWM01S directories)

3. LOAD NEW V28-SPECIAL
Go to car, turn ign on, wait for mono nav (pre-loaded with v24) to boot (2 min to be sure). Remember, you won't see anything on the monitor yet.

Insert v28-special OS CD

Nav will load new v28 software, when done, will eject CD.

Remove CD and press the bordmonitor button

Nav will re-boot. When it starts, the colour screen will come.

Cheer loudly :-)

The nav will boot with blank telematics settings (default = 7er limousine, no emergency call, no vin loaded) and also with default languages (D, GB, F)

If you want, use NavTweak to load correct VIN and vehicle/country/widescreen type into nav, and set new languages.

Use the v28-special for the new languages (although as it's only loading language files, you can most likely use a normal v28 disc)

Enjoy! Make sure you post your successes back here on the Forum

Cheers, Jochen