@Lou as you say it shouldn’t effect the grant at all. Unless its a special colour then your liable for the difference. Example the metallic is £500 but they do a special red £700 so you’d have to pay the difference of £200. This would count as an extra regards the grant.
What car and colour have you chosen if you don’t mind me asking?
Current Car
KIA Sportage GT Line Mhev
Previous Cars
Peugeot 5008 Allure Premium
VW Touran SEL
Citreon C4 Picasso