To amend the vehicle colour on a stock record, the vehicle must be un-adopted.
If the vehicle is adopted, this will lock the colour in. To un-adopt a vehicle, please go into the stock record> Purchase Status> Un-Adopt (A pop up box with the date will then appear) click adopt again.
Go back to the details tab and next to the colour box (highlighted below) an 'Update colour' button will appear, click this and either select your colour from the drop down list or add a new colour by clicking the + button then update.