Airbus does not like many customizations in the final paint of components that are in the same color for the majority of airlines. For example the engine nacelles and wings. Each request is evaluated “case by case”.
But some airlines have their image linked to a color and British Airways (as United) will have the engine nacelles painted in the carrier's traditional blue.
If all the fleet have blue nacelles, the A350 XWB will have them blue painted keeping with the scheme on the rest of its fleet.
The first A380 for British has blue nacelles.
Boeing was also considering the same issue in British Airways´ 787 that finally have had their nacelles painted in blue. "We need to evaluate each custom color request because different colors may require different thicknesses to achieve the desired appearance," Boeing said.
"We need to ensure that the thickness required does not exceed the design tolerance."
The paint is crucial to the aircraft's economics because laminar flow reduces skin friction drag, and cuts fuel burn, but it requires a smooth surface with minimal boundary disruption.
Even the thickness of paint layers can interrupt the laminar flow between the engine inlet and the surface of the nacelle.
Based on the article “British Airways 787 emerges with blue nacelles” published in Flightglobal.