Remove phone from bluetooth handsfree car unit's...

Sorry but what do you mean by?:-

'The car unit will not allow me to use the colour previously used'

It should be the phone with the memory and the search facility for finding your car headset device, not the device looking for the phone.

Once you have searched for new devices and found the headset with a different phone, the headset will not be able to connect with more than one device and should lose the old phone connection.