My Dad worked as a Motability specialist for several dealers over the years and according to him, Motability customers can choose any colour they like without charge regardless of how expensive it is. His advice was to speak to Motability who could then remind the dealer of this policy.
Unfortunately your dad is wrong! Indeed I had an argument I lost once where Motability would not let me order a car with the cheapest colour available (hard to believe but linked to AP and residual value)