A non-Alfa color, or a non-period correct color (even if it is an Alfa color) will always negatively affect the value of a concours car. Otherwise, it is a matter of taste. It does seem to be the case nonetheless that once a car is customized, and the owner wants to sell it, it is much harder to find a buyer whose taste matches the customized choice. Red, however, is pretty popular, so if it is a driver example, it probably won't make much difference. What does make a difference is the execution of the color change, or lack thereof. If the color is not changed throughout, and the engine bay, trunk, door jambs are not re-painted, then it will always be perceived as an indication that the re-spray was done on a budget and leave potential buyers to wonder about the general quality of work that the particular car has received. An exquisite paint job however, will always show through, regardless of how original or correct the chosen color is or isn't.
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