It’s what a lot of manufactures do now in that they have free of charge flat paint, extra cost metallic paint and extra charge on top of the extra for metallic for premium paint and Motability pay for metallic paint but won’t pay for the premium paint.
What your dealer is asking for, or should be asking for is the difference between the cost of metallic paint (that Motability pay for) and the cost of the premium paint assuming of course that colbart blue is a premium paint as I don’t know.
My guess why its only just been brought to your attention is because they have only just put the order into the manufacture and its come to light but you really should have been told at order point and just wonder what your order invoice says.