The colour of the fleet numbers can be changed in the model config files for the buses. These are found in London Citybus 400\Model folder and the default files are named "model_ADL_R.cfg" and "model_ADL_R_Express.cfg".
Open them in notepad and find the section that looks like this:
View attachment 75173
The highlighted values correspond to the RGB values that the number will take, so change these to the RGB values of your desired colour. This will change the colour for all of the fleet numbers on the bus, you won't be able to change them individually.
As for your second question, it's not possible to change the location of the fleet numbers as they're part of the model.